GB2524036A - Erosive wear testing apparatus and method - Google Patents

Abstract

A method of carrying out erosion wear testing of a test sample includes providing an exhaust gas stream of a jet engine through pipe 22, introducing erosive wear particles into the exhaust gas stream through particle feed pipe 32 and directing the exhaust gas stream containing the erosive particles through an exit nozzle 26 onto the test sample. The test sample may be mounted on a motorised specimen holder with other samples. The test apparatus provides temperatures and particle velocities similar to those in operational engines.

Description

Erosive wear testing apparatus and method

Technical Field

The present invention relates to a method of carrying out erosion wear testing of a test sample and an erosive wear test apparatus.

Background and Prior Art

Aircraft gas turbine engines operate in very harsh environments, being exposed to severe mechanical loads and high temperature. Erosion caused by high speed particles can be an additional (and serious) issue in aircraft gas turbine engines, especially when flying through desert environments, as the ingestion of sand (solid particle) is inevitable. Protective coatings are usually applied to aircraft engine parts to enhance their durability, reduce fuel burn and maximise the exploitation of properties provided by commercial turbine materials.

Therefore, a method for testing the erosive wear properties of different turbine materials and protective coatings is of great interest. One difficulty arises from conceiving a testing method/facility/rig that will closely reproduce in-service conditions found in aircraft engines (i.e. high temperatures and high particle velocities). Moreover, the ideal' erosion testing method should be easy to perform, reproducible, quick (i.e. not time consuming) and cost-effective (relatively inexpensive).

Although many high temperature erosion testing facilities exist, only a few can closely simulate high temperatures in addition to high erosive particle velocities as those found in aircraft gas turbine engines. It is known that aeroengine blades are exposed to temperatures of up to 1100C° and relative velocities between particles and blades normally exceed 100 m/s and can often approach 300 m/s.

Several high temperature laboratory erosion test rigs have been proposed, especially to investigate erosion-corrosion issues in fluidised bed combustors (FBCs). Temperatures up to 850°C are often achieved but the velocity of impacting particles is in the order of 0.2-40.0 mis, being considerably lower than those encountered in aircraft gas turbine engines (hundreds of meters per second).

Several high temperature erosion rigs have been used to investigate the oxidation-corrosion-erosion performance of various materials or protective coatings for a number of different operating conditions such as those encountered in coal-fired boilers or gas turbine engines for power generation and aerospace applications. Variations can be found in the way the gas stream is heated up (normally via electrical furnaces or combustion). Hot gas is used to accelerate injected particles along a tube to impact test specimens. The difference between high temperature gas jet and gas gun rigs relies on their mode of operation. In gas jet rigs the erodent particles pass through a relatively short acceleration tube and the particle feeder, acceleration tube and specimen stage are often contained within a single furnace system. In gas gun rigs the acceleration tube is much longer (often up to several meters) and there are separate gas, particle and specimen heaters. Single and multiple particle impacts can usually be investigated using gas gun rigs as well as continuous erosion testing. These rigs are physically large and relatively expensive to build.

The main disadvantage of high temperature gas gun rigs is that erosion testing cannot commence until the transport gas has been uniformly heated and reached the desired testing temperature. However, they do offer a broad range of test conditions (temperature, erosive particle velocities, impact angles, etc).

Although gas jet rigs are much more compact in size compared to gas gun rigs, the maximum erodent particle velocities achieved in such rigs is of the order of tens metres per second and, therefore, much inferior to velocities found in aircraft turbine engines (in the order of hundreds of metres per second).

High temperature combustion (burner) rigs have also been used to investigate the erosion behaviour of materials and coatings at high temperatures.

Shimizu et. al (Wear 267, 104-109 (2009)) also used a high temperature, high velocity erosion rig to perform erosion tests up to 900°C. Reported particle velocities were 226 rn/s and 100 mis. This rig also relies on an electric furnace to heat the gas stream and erodent particles.

Aero-engine manufacturers and military end-users also perform high temperature, high velocity erosion tests that closely simulate in-service conditions. These "rainbow engine sand ingestion test" facilities employ a full-scale gas turbine engine which is subjected to a stream of sand ingestion. Such tests can be found at General Electric (Z.J. Przedpelski, Journal of the American Helicopter Society 29, 63-69 (1984)), the US Air Force Base at Kirkland (W. Tabakoff and G. Simpson, Study of CVD and PVD Coatings for Turbomachinery Life Predictions. In: Proceedings of the 22Id Heat Treating Society Conference and the 2nd Internationa/Surface Engineering Congress, pp.453-462, ASM International, Materials Park (2003)) and at the US Naval Air Station Patuxent River (NASPAX River) in Maryland (B.L.

Fischer, M.A. Klein and V.M. Cintrón., Erosion Durability Improvement of the T58 Engine for Military Helicopters. In: Forum 61 Proceedings, The American Helicopter Society, Grapevine (2005)). Although these tests closely replicate in-service conditions, both construction and operational costs are very high and tests are time consuming.

A miniature (compact) jet engine has been used by Vextec (US patent 8,006,544 B2) to perform small-scale engine tests that simulate conditions of a full-scale gas turbine engine and its failure mechanisms. Tests are carried out to investigate several material failure mechanisms such as thermomechanical, creep and cracking fatigue as well as stress corrosion, weld repair and foreign object damage. Materials are tested by removing an original component (e.g. blades) from the miniature test engine and substituting a new component in place of the original component.

High temperature erosion devices are also disclosed in patents CN101413860B, CN10147701A, CN103063534A, CN201340370Y, CN103091237A and JPH0862114A.

Summary of the Invention

In a first aspect, the invention relates to a method of carrying out erosion wear testing of a test sample, the method involving providing an exhaust gas stream cia jet engine, introducing erosive wear particles into the exhaust gas stream and directing the exhaust gas stream containing the erosive particles onto the test sample.

In a second aspect, the invention relates to an erosive wear test apparatus, the apparatus comprising a jet engine capable of generating an exhaust gas stream, means for introducing erosive wear particles into the exhaust gas stream in use, wherein a test sample can be positioned such that the exhaust gas stream containing the erosive wear particles can be directed onto the test sample.

The use of a jet engine as the source of the hot air stream together with a supply of erosive particles makes possible a particularly convenient test apparatus and method. The jet engine provides the hot gas stream, that accelerates the erodent particles that will then impact on the test specimen surface. Therefore, no separate heated gas stream, acceleration and specimen systems are required. This new testing methodology is quick, easy to perform and inexpensive compared to high temperature, high velocity erosion rigs in the prior art. It also provides means of scaling down high temperature, high velocity erosion testing facilities.

Moreover, the temperature and erodent velocity are close to conditions encountered in aerospace gas turbines.

It has been found that even a relatively small jet engine is capable of providing the required velocities and temperatures to adequately capture the conditions encountered in aerospace gas turbines. Furthermore, the smaller the jet engine the less costly is the method and apparatus. Thus a jet engine with a mass of less than 100kg, more preferably less than 50kg, still more preferably less than 20kg, still more preferably less than 10kg, or even less than 5kg or less than 3kg, i.e. from 0.5 to 3kg, are preferred. However, it must be borne in mind that a larger jet engine can provide higher temperatures and erodent velocities.

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The hot air exhaust stream has a temperature of from 500 to 1200°C, more preferably from 800 to 1100°C, depending on factors such as jet engine size and power rating.

The apparatus can have more than one jet engine, such as from two to ten, however one jet engine is perfectly acceptable.

The test sample can be any suitable test sample. However the present invention finds particular utility in simulating the environment experienced by aero engine components such as compressor blade materials. The test samples can be flat coupons or discs as well as actual engine components (e.g. compressor blades).

Additionally, the apparatus and method can be used to test the effectiveness of various coatings to evaluate their effect on erosive wear.

Preferred erodent particles are alumina and silica (sand). The particle size of the erodent particles can be any desired particle size, according to the needs of the testing environment.

For example the present invention works equally well when testing for the impact of fine particles as well as for coarse particles.

In one preferred embodiment the apparatus and method comprise a motorised specimen holder which allows sequential testing of up to six specimens whilst the jet engine is running.

In a preferred embodiment the erodent particles are fed into the exhaust gas stream via a delivery pipe.

The particles are typically entrained in a carrier gas, such as an inert gas, for example a noble gas or nitrogen. Alternatively they can be carried in compressed air.

The method and apparatus are capable of providing erosive particles with a test sample impact velocity of from ito 300 mIs, preferably from 2Dm/s to 300m/s, more preferably from 50 m/s/ to 300 m/s. The precise velocity will depend on a number of factors, including the jet engine size and power as well of the nature of the erosive particles.

To provide laminar flow in the exhaust gas and a repeatable focussed stream of gas, it is preferable to reduce the effective diameter of the exhaust gas stream by passing it through a narrower bore pipe before exiting to travel towards the test sample.

Therefore, this new erosion method described offers a reliable and quick possibility to investigate the oxidation-erosion behaviour of different materials and protective coatings for a variety of test samples, e.g. compressor blade applications.

When compared to other high temperature, high velocity erosion testing methods available, elevated temperatures are achieved almost instantaneously without any requirement for separate heating of specimens and gas stream. Likewise, long acceleration tubes usually found in high temperature erosion test rigs such as gas gun rigs (to provide high erodent velocities and also ensure that thermal equilibrium is reached between the erodent particles and gas stream) are not essential, which allows for the erosion test apparatus to be made very compact in size.

Tests can commence immediately after the jet engine is set up and running. This testing method can be used to rank the erosion resistance of materials and/or protective coatings, such as metal nitride coatings, under specified testing conditions.

Detailed Description of the Invention

The invention will now be described by way of example, and with reference to the following figures, in which: Figure lisa photograph of a miniature (compact) JetCat 160-SX jet engine used in this invention.

Figure 2 is a schematic of the jet engine exhaust pipe comprising a primary pipe, cone and exit nozzle (design of exhaust gas pipe).

Figure 3 is an inset of the gas exhaust pipe showing that erodent particles enter the gas stream at a 30° angle.

Figure 4is a photograph of the high temperature, high velocity erosion rig showing the Venturi pipe and blast head used to deliver the erodent particles to the jet engine exhaust pipe.

Figure 5 is a schematic of the motorised multi-specimen holder. The impingement angle of erodent particles can be varied from 1° to 90°. Although not shown here, this system can hold and test up to six specimens whilst the jet engine is running.

Figure 6 are SEM photomicrographs of the CrAIN-coated AISI M2 steel specimen surface prior to (a) and after (b) erosion tests using the high temperature, high velocity erosion rig described in this invention. [rodent feed rate, test duration and erodent impingement angle were 2g/min, 2 minutes and 30° respectively. CrAIN coating thickness was 2.0 ± 0.1 m.

The novel test rig can be divided into four main parts: (i) Jet engine 10 (source of hot gas stream and erodent acceleration); (ii) jet engine exhaust pipe 20 in which erodent particles are injected into the gas stream to accelerate towards the specimen; (iii) erodent particle feed system 30 and (iv) motorised specimen holder 40 which allows testing of up to six specimens whilst the jet engine is running.

The jet engine 10 used to demonstrate this invention was a miniature (compact) JetCat 160-SX jet engine (Fig.1). The nominal thrust and exhaust gas temperature are 16kg at 125000 RPM and 650-750°C (data provided by engine manufacturer, see Table 1). Its small dimensions and low fuel consumption make it ideal for a compact erosion test rig which will deliver high temperature and high erodent velocities similar to those found in aircraft turbine engines. As previously mentioned, higher temperatures and erodent velocities could be achieved by using larger jet engines.

The jet engine exhaust pipe 20 is shown in Fig.2. It consists of a 78 mm inner diameter pipe 22 (465 mm long), onto which a reduction cone 24(60 mm long, 60 mm diameter) was attached, connected to a 10 mm inner diameter exit nozzle 26(250 mm long, 10mm inner diameter). This approach ensured that a laminar flow was achieved (and hence well-defined, repeatable circular erosion scars characterised by a small degree of scattering always resulted on specimen surfaces).

The erodent delivery pipe 30 is illustrated in Fig.3. [rodent particles travel from the feeding system through a 6mm innerdiameter pipe 3Zto enterthe jet (gas) stream at the cone part of the exhaust gas pipe. The erodent particles enter the jet (gas) stream at an angle of 30°.

Table 1: Technical data for the JetCat 160-SX jet engine (data provided by engine manufacturer).

Thrust 16kg at 125,000 RPM Weight (kg) 1.4 Diameter (cm) 11.2 RPM range 32,000 to 125,000 [xhaust gas temperature (C) 650-750 Fuel 1-K kerosene, Jet Al Fuel consumption at full power 17 (ohm in) Lubrication approximately 5% synthetic turbine oil in the fuel Maintenance interval (hrs) 25 The erodent feed system allows the feed of a known mass (usually quantified in grams) of erodent particles into the hot gas stream at a carefully controlled rate. [rodent particles are transported to a blast head by a Venturi pipe into which a gas is admitted to deliver the erodent particles to the exhaust gas pipe (Fig.4). Any noble gas, compressed air or nitrogen can be used; in this present invention nitrogen was used. This example uses a CH2 blast head made from aluminium and manufactured by Anglo Scot Abrasives.

Specimens to be erosion-tested are placed at the front of the exit nozzle. Although the working distance (i.e. distance from specimen surface to exit nozzle) can be varied (and hence the testing temperature and erodent velocity), this parameter was set to 30 mm to demonstrate this present invention. Specimens can be flat coupons, discs or any component (e.g. compressor blades). The motorised multi-sample holder is illustrated in Fig.5.

Specimens 42 can be mounted at any erodent impingement angle (i.e. angle between the nozzle axis and specimen surface) varying between 1° and 900.

The motorised specimen holder can be programmed to expose each specimen to the jet stream and erodent particles sequentially in one erosion test session. This provides a significant benefit over the existing larger high speed high temperature erosion rigs in the prior art where the system has to be cooled between test samples making the testing process slow compared to the current invention.

Examples

Determination of erosion testing temperature The temperature of the exhaust gas from the jet engine was measured at the end of the exit nozzle. A type K thermocouple with inconel sheath (1.5 mm diameter, 1.2 m long) was used for this purpose. The measured temperature at the exit of the nozzle when the jet engine was at full throttle (i.e., 125,000 RPM) was 890°C. Therefore, it is assumed that the temperature of the specimen to be tested would be very close to 890°C, as the distance from the nozzle to specimen surface is set at 30 mm.

Determination of erodent particle velocity The velocity of erodent particles was measured using a high-speed, high-resolution camera (Photron FASTCAM Mini UX100). Recordings were taken at a 30,000 frames per second. The erosive media used was pink fused alumina (McAnts Alumac (A1203) 180/220 grit, average particle size of 53-74 rim). A Maximum erodent velocity of 128 mIs was measured when the engine was at full throttle (i.e. 125,000 RPM). It is envisaged that higher erodent velocities could be achieved by reducing the erodent particle size. Smaller particles should reach velocities closer to that of the hot gas stream. Likewise, the use of a larger jet engine could also increase the maximum erodent velocity.

Example of an eroded specimen using the high temperature, high velocity erosion rig described in this invention A polished, hardened AISI M2 test disc (29.5 mm in diameter, 5 mm thick) coated with a CrAIN coating deposited by PVD (Physical Vapour Deposition) was erosion-tested using the high, temperature, high velocity erosion rig described in this invention. The CrAIN coating thickness was 2.0± 0.1 Jtm and the arithmetic average roughness (R2) of the tested disc specimen was 0.04 ± 0.01 Iim. The erosive media was pink fused alumina (McAnts Alumac (A1203) 180/220 grit, average particle size of 53-74 iim) and the erodent impingement angle was 30°.

The erodent (particle) feed rate was set to 2 g/min and the test duration was 2 minutes (total mass of erodent alumina was 4 g). An elliptical scar resulted on the specimen surface due to the 30° impingement angle.

The surface morphology prior to and after erosion (Fig.6) was evaluated by scanning electron microscopy (SEM). The eroded specimen surface was characterised by small indentations with little or no evidence of plastic deformation, suggesting a mechanism of erosive wear by brittle fracture.

Claims (14)

1. Claims 1. A method of carrying out erosion wear testing of a test sample, the method involving providing an exhaust gas stream of a jet engine, introducing erosive wear particles into the exhaust gas stream and directing the exhaust gas stream containing the erosive particles onto the test sample.
2. 2. An erosive wear test apparatus, the apparatus comprising a jet engine capable of generating an exhaust gas stream, means for introducing erosive wear particles into the exhaust gas stream in use, wherein a test sample can be positioned such that the exhaust gas stream containing the erosive wear particles can be directed onto the test sample.
3. 3. A method or apparatus according to any one of the preceding claims, wherein the temperature of the exhaust gas stream is from 500 to 1200°C.
4. 4. A method or apparatus according to any one of the preceding claims, wherein the erosive wear particles have an exit velocity of from ito 300 mIs.
5. 5. A method or apparatus according to any one of the preceding claims, wherein the jet engine has a mass of less than 100kg, more preferably less than 50kg, still more preferably less than 20kg, still more preferably less than 10kg, or even less than 5kg or less than 3kg, i.e. from 0.5 to 3kg.
6. 6. A method or apparatus according to any one of the preceding claims, wherein the erosive wear particles are transported to a blast head by a Venturi pipe to the exhaust gas stream.
7. 7. A method or apparatus according to any one of the preceding claims, wherein the erosive wear particles are carried by an inert gas, preferably a noble gas, to deliver the particles to the exhaust gas stream.
8. 8. A method or apparatus according to any one of claims ito 6, wherein the erosive wear particles are carried by compressed air to deliver the particles to the exhaust gas stream.
9. 9. A method or apparatus according to any one of the preceding claims, wherein the exit to the jet engine has a size reduction nozzle.
10. 10. A method or apparatus according to any one of the preceding claims, wherein the test sample is a flat coupon or disc or an aerospace component, such as compressor blades.
11. 11. A method or apparatus according to any one of the preceding claims, wherein the erosive particles have a test sample impact velocity of from ito 300 mIs.
12. 12. A method or apparatus according to any one of the preceding claims, wherein the test sample is coated with a protective coating, such as a metal nitride coating, forexample CrAIN.
13. 13. A method or apparatus according to any one of the preceding claims, wherein a plurality of test samples are arranged for testing on a motorised specimen holder.
14. 14. A method or apparatus according to any one of the preceding claims, wherein the erosive particles are selected from alumina and silica.