GB2504527A - Method and apparatus for testing a sample surface - Google Patents

Abstract

A method for testing the anti-fouling properties of a sample surface involves mounting a sample 212 in a holder 224; mixing a contaminant with an air flow to provide contaminated air 280; supplying the contaminated air to a delivery device; projecting the contaminated air onto a surface of the sample 212 using the delivery device; and, weighing the sample to ascertain the amount of deposited contaminant. A sample chamber holds the sample and delivery nozzle 220. A delivery device includes three sources of contaminant 226, 228a, 228b, to mix with air 230 to provide the contaminated air 280. The sample holder 224 is adjustable to vary the angle of incidence of the air 280 on the sample. The method may also include cleaning the sample to test removal of contaminants.

Description

A Method and Apparatus for Testing the Surface of a Sample

Technical Field of Invention

This invention relates to a method and apparatus for assessing the surface of a sample. In particular, though not exclusively, the invention relates to a method and apparatus for assessing coatings which are used on gas turbine engines. Of particular interest to the invention is the assessment of anti-fouling coatings.

Background of Invention

The treatment and preparation of components to achieve desired surface properties is well known. Such treatments include the use of coatings to improve the surface properties of component. Coatings can include simple systems, such as paint to protect a metal object from oxidisation, or more sophisticated coatings which provide thermal shielding or abrasion resistance in harsh environments, for example.

One group of coatings are the so-called anti-fouling coatings which help prevent the build up of deposits on the surface of a component. The type of deposits is application specific, but taking the aero-industry as an example, such deposits may include airborne contaminants such as soil, smoke, oil, salt, water and pollen, to name a few. Examples of general anti-fouling coatings which are used in the aero-industry include commercially available coatings such as SermaLon/Sermaflow 54000 produced by Sermatech and PX-2000 coating produced by Praxair.

However, there is no technique or apparatus available to the applicants for assessing these coatings. And more particularly, there is no guidance as to suitable tests which can provide a testing methodology which can mimic a period of service for gas turbine engine.

The present invention seeks to provide a test methodology and apparatus which can be used to assess anti-fouling coatings.

Statements of Invention

The present invention relates to a method of testing the antifouling properties of a sample surface, comprising: mounting a sample in a holder; mixing a contaminant with an air flow to provide contaminated air; supplying the contaminated air to a delivery device; projecting the contaminated air onto a surface of the sample using the delivery device; and,weighing the sample to ascertain the amount of deposited contaminant.

Providing a method as described above, it is possible to ascertain the amount of deposited contaminates on the surface of the sample. Further, the method of the invention allows for the deposition of contaminates to be readily controlled by adjusting the mixing rate of the first contaminate and airflow rate.

By anti-fouling properties it will be appreciated that this relates to the attributes of a component which allows it to resist adhesion of certain particles or other contaminants. It may also refer to the ease of removal of such deposits which do adhere to a surface. The anti-fouling properties may be attributes of the sample material or its surface finish or treatment, or attributes of an additive coating or membrane which may or may not act in conjunction with the material.

The relative position of the sample holder and delivery device may be adjustable. The method may further include the additional step of adjusting the angle of incidence at which the contaminated air is projected on to the sample.

The relative position of the sample and delivery device may be such that the position and angle at which the contaminated air is projected onto the sample can be adjusted. Further, the distance between the point of projection on the delivery device and the sample may be

adjustable.

The method may further comprise the step of mixing a first and second contaminant with the air supply to provide the contaminated air.

At least one of the first and second contaminant may be a liquid, wherein the mixing of the liquid with the air flow includes introducing the liquid into the airstream via an atomiser.

There may be a plurality of contaminants. The contaminants may be liquids, solids, suspensions or solutions or a mixture of these. By solid it will be appreciated that the material may be a particulate such as a powder. The powder may be a mineral powder. The solid material or particles thereof may be porous. The projection of the contaminated air may be a spray. The spray may be from a nozzle.

The method may include the steps of providing a rate of deposition which is accelerated compared to the normal expected deposition rates for the component.

The deposition rate may be selected to be between 500 times and 1000 times the normal accretion rate. Preferably, the rate of deposition is between 600 times and 900 times the normally experienced accretion rate.

The method may further comprise cleaning the sample for a predetermined time and weighing the sample to establish how easily the fouling can be removed.

The cleaning and weighing steps may be repeated. The cleaning may be a warm water ultrasonic clean. The warm water may be in the region of 40 degrees centigrade.

The method may further comprise steps of: analysing the weight data provided by weighing the samples pre-deposition, post-deposition and post-cleaning and, determining a fit for use factor.

The fit for use factor may take the form of C+K/L., where C is the deposited amount of contaminant before cleaning, K is the amount of post-cleaning contaminant, and L is the reduction in rate deposit reduction for a given cleaning time.

At least one contaminant is a powdered material having a particle size below 10 microns in diameter.

The sample may be representative of a gas washed surface in a gas turbine engine.

In a second aspect, the present invention provides an apparatus for testing the antifouling properties of a sample surface comprising: a sample holder for holding a sample to the tested; mixing apparatus for mixing an airflow with a contaminant so as to provide contaminated air; and, a delivery system for projecting the contaminated air on to the surface of the sample.

The apparatus may further comprise a plurality of mixing apparatuses.

The delivery system may include an atomiser for creating an aerosol from a liquid contaminant and the airflow.

The sample holder may be adjustable such that the angle of incidence between the surface to be coated and the direction of the projected contaminated air can be adjusted to a predetermined angle. The sample holder may include a rotation mechanism of pivotable arrangement.

In a third aspect, the present invention may provide a system for testing the antifouling properties of a sample surface, comprising the apparatus of the second aspect and a weighing apparatus.

The system may further comprise a cleaning apparatus for cleaning the surface of the sample.

Description of Drawings

Embodiments of the invention will now be described with the aid of the following drawings of which: Figure 1 shows a flow diagram of the method of the present invention.

Figure 2 shows an apparatus for depositing contaminated air on to a sample.

Figure 3 shows an atomiser.

Figure 4 shows a plot representing two cleaning cycles for two samples.

Detailed Description of Invention

Figure 1 shows a flow diagram which illustrates a method of testing the antifouling properties of a sample surface 10 according to the invention. The method generally includes the steps of mounting a sample in a holder 12, mixing an air supply with one or more contaminants to provide contaminated air 14, supplying the contaminated air to a delivery device, projecting the air and contaminants onto a surface of the sample 18 and weighing the sample to ascertain the amount of deposited contaminate 20. In some embodiments, the method also involves cleaning the sample and re-weighing to assess the ease of removal of contamination.

[ach step of the method is described in detail below, after a description of the apparatus used to mix and deposit the contaminated air and project it on to the surface of the sample, as shown in Figure 2.

Figure 2 shows an apparatus 210 for depositing a contaminant on to a sample 212. The apparatus includes a chamber 214 and a delivery system. The delivery system includes an arrangement of feeding devices 216, 218 and various interconnecting conduits which provide contaminated air to a delivery device which is in the form of a nozzle 220 located within the chamber 214. The nozzle 220 is arranged to project the contaminated air toward a sample 212 which is located within a sample holder 224, which is also located within the chamber 214 at an appropriate location.

The delivery system includes three sources of contaminant 226, 228a, 228b and an air supply 230 into which the contaminants are mixed via the respective feeding devices 216, 218 and conduits prior to being deposited on the sample 212. Thus, there is provided an aerosol system in which an air supply is loaded with the desired contaminants before being sprayed onto the surface of a sample 212.

The air supply 230 may be any suitable type which can provide the necessary pressure and mass flow rate to achieve the desired mixing and spray action as required by the application. In the described apparatus, the air is taken from a compressed air supply 230 which is delivered through an appropriate control scheme (not shown) which can adjust the pressure and flow rate as required.

The first contaminant 226 is in the form of a solid particulate material, such as a mineral powder, which is introduced by a particle delivery apparatus in the form of a rotating particle feeder 216. In the present embodiment, the particulate delivery apparatus 216 includes a housing 236 having an inlet 238 and an outlet 240 and an airflow path 242 therebetween.

The airflow path 242 may be provided by a conduit or passageway which is internal to a housing 236, or may simply be confined by the walls of the housing 236 itself as shown in Figure 2. The rotating particle feeder 216 is loaded with the solid material in the form of particulate matter and arranged to introduce the particulate matter into the air flow path 242 via an opening 244. The rate of release into the airflow is controlled by adjusting the rate of rotation which is controlled via the motor 246 and a speed controller 248. It will be appreciated that various control devices may be implemented to control the flow of particulate material. However, a rotating particle feeder 216 is particularly advantageous due to the level of release rate control which can be achieved.

The outlet 240 of the particulate delivery apparatus 234 is connected to a conduit in the form of a flexible pipe 250 which connects to an inlet 252 of the sample chamber 214.

The sample chamber 214 includes a housing 254 which provides an enclosed space 256 in which there is located a sample holder 224, a delivery nozzle 260, a second 262 and a third 264 contaminant delivery device, and a flow collimator 266.

The airflow path extendsfrom the inlet 252 of the chamber 214 to the delivery nozzle 260 in the form of a conduit 268. The conduit 268 passes through the collimator 266, which is in the form of a flow straightening orifice plate and which acts to straighten the non-laminar flow which enters the chamber inlet 252 from the particle delivery apparatus 234.

The delivery nozzle 260 is in the form of a convergent nozzle which acts to accelerate the pressurised air prior to it being ejected towards the sample 212 which is held in the sample holder 224 below. The nozzle 260 also includes an atomising portion 270 for introducing liquid contaminates to provide the aerosol for deposition.

The second 272 and third 274 contaminants are in liquid form and are introduced into airflow via the atomising portion 270 so as to form an appropriate aerosol having the desired attributes. It will be appreciated that the there could be more or less contaminants which will be application specific.

The delivery systems of the second 272 and third 274 contaminants include individual piston delivery systems 276, 278 in the form of plunged syringes. The plunged syringes are controlled using syringe pumps so as to deliver the required rate of liquid as known in the medical arts for intravenous delivery of medicaments, for example. Individual tubes connect the syringes to the airflow path of the delivery system such that the liquid contaminants 272, 274 are introduced into the air flow at the convergent nozzle 260, via the atomiser, as shown in Figure 3.

Thus, in Figure 3 there is shown the atomiser 270 having two small bore metal tubes 312 314 which each receive one of the liquid contaminant from the syringe system. Each of the metal tubes protrude into the airflow path 316 in the convergent portion of the nozzle 260 on opposing sides of the airflow and normal thereto, such that exiting droplets 318, 320 are atomised by the high shear air flow experienced. The shearing force causes the liquid flow to fracture into fine droplets in the air stream, thereby providing converging cones 322, 324 of contaminated air which intermix into a single contaminated flow 326. In the embodiment, the small bore metal pipe inner diameter of approximately 0.3mm.

Referring back to Figure 2, the sample holder 224 is arranged to be in the flow path of the contaminated air 280 which projects from the delivery nozzle 260 and is spaced at an interval from the nozzle 260 to allow the contaminated flow to diverge to a desired size. The sample holder 224 includes a holding arrangement 282 which is configurable to receive a sample 212 for receiving the contaminated air flow 280. It will be appreciated that the holding arrangement 282 may be any suitable device for retaining the sample 212 and may include a clamp, ties or a number of bolts for example. In the described embodiment, the sample 212 is received within a recessed portion of the sample holder and retained there with a magnetic holding device 284. The magnetic holding device includes magnets 286 which are fixed to the sample holder 224 and a ferromagnetic backing plate 288 for interaction with the magnets 286. A magnetic holding device 284 is particularly advantageous as it allows for a suitable restraint without obscuring any of the face areas of the sample which may affect the results. It will be appreciated that other methods of surface contact retention devices, such as adhesive tape or hook and loop fasteners, may be implemented to the same effect.

The sample holder 224 is mounted to sample chamber housing 254 via an adjustable stand 290. The adjustable stand 290 includes a rotation mechanism 292 which is adjustable relative to the delivery nozzle 260 such that angle of incidence at which the contaminated flow hits the sample 212 can be altered. This is particularly advantageous when considering applications for gas turbine engines where the angle of airflow is different for different components in normal use. Further, some components experience varying flow conditions during a flight cycle due to mechanical variations or adjustments made during flight and different parts of the flight cycle may result in different angles of attack of contaminants.

It will also be appreciated that as well as providing rotational adjustment, the sample holder 224 may also be movable relative to the delivery nozzle 260 in an x-y reference frame to allow a particular area of the sample 212 to be aligned with the contaminated flow. It will be appreciated that by moveable relative to, it is meant that either or both of the sample holder 224 and nozzle 260 may be moved with respect to the chamber 214 to provide the relative movement required. Further, the sample 212 may be moveable within the sample holder 224 to the same effect.

In addition to the above described features, the sample chamber 214 also includes a number of other ancillary features including: an exhaust 296 for exhausting the contaminated air 297; a viewing portal in the form of a window or possibly a camera so that the process can be observed; an opening for loading the sample which is sealable so as to be substantially airtight and a pressure release valve 298. As will be appreciated, the contaminated air exiting the exhaust port may be treated appropriately before being discharged into the environment. For example, a working system may incorporate some form of wet-scrubber tank and local exhaust ventilation.

In the described embodiment the sample 212 is representative of a component from a gas turbine engine. The component can be any one which has a gas washed surface and which suffers from contamination. For example, an outlet guide vane, fan blade, compressor blade or portion thereof.

Contamination of gas washed surfaces in gas turbine engines results from airborne particulates which accrue on the surfaces as they pass over them. The contaminants experienced by gas turbine engines typically include soil, smoke particles, oil, salt, water and pollen etc. Such particles are found in different sizes and concentrations around the world and vary as to the local environment. As an example, above Antarctica, typical levels of contaminants having a particle size less than 10 microns in diameter is typically 3.4 jig/m3.

In the airspace above a forest fire, the levels experienced by a gas turbine may be in the region of lmg/m3. Hence, the amount of accretion depends on multiple factors including but not limited to the geographical area of service and the particular flight cycles experienced by the engine in question. This makes effective testing of coatings particularly onerous and one which, in some instances, is quite particular to the anti-fouling coatings for the aerospace industry.

In order to account for the variance in possible environments and to simulate a reasonable lifecycle for an engine, high concentrations of particles are used to bombard the sample.

The level of contaminants may be between 0.Sg/m3 and lgJm3 which represents between 500 and 1000 times the upper level of contamination which may be experienced in service.

Ideally, the level of concentration is held to be approximately 24000 times what might ordinarily be expected in a moderate flight cycle, or 600 times the worst case scenario. With these sorts of deposition rates, the applicants have found that accretion rates can be increased to around 900 times the normally experienced rates, thereby providing an accelerated way of effectively field testing the components.

The contaminants used in the described embodiments included a mineral oil having additives in the form of carbon black (16% by weight), ferric oxide (2% by weight) and PV resin (4% by weight), a dust to simulate dirt and soot etc in the form of ISO standard dust 12103, Al grade, and sea water to provide an accretion of salt and minerals on the component.

Prior to commencing a test, the sample 212 is coated with the desired coating using an appropriate method, as dictated by the product. Once coated, the sample 212 is cleaned, typically using a warm water ultrasonic bath, and dried. It will be appreciated from the above description, that the preparation of the sample 212 may also include the attachment of the sample material to a substrate or other material as may be required for mounting the sample in the sample holder. The substrate or other material may include a ferromagnetic plate 288 or an adhesive strip etc. Once prepared, the sample 212 is weighed to ascertain the pre-deposition weight. This weight is recorded such that it can be compared against the post-deposition weight of the sample 212, thereby providing a baseline against which the amount of contamination can be assessed.

Once weighed, the sample 212 is loaded into the sample holder 224 of the chamber 214, and the sample holder 224 adjusted to place the sample 212 in the correct position relative to the nozzle 260 and at the correct orientation to provide a desired angle of incidence for the contaminated air 280. The door of the chamber 214 can then be closed ready for the coating operation.

The contaminants are loaded into syringes 262, 264 and hopper with the desired feed rates calculated and programmed into the respective controller. The air supply 230 pressure and flow rate can also be calculated and pre-adjusted, if applicable, so that a desired nozzle exit speed is achieved. The relationship between the air inlet pressure and nozzle outlet speed may be known from empirical data or from pre-operation calibration which occurs prior to the sample 212 being loaded.

Once these steps have been taken the air supply 230 can be switched on and ramped up until the desired output speed is achieved.

The contaminants can be introduced during different intervals, either individually, or together. For example, the delivery of the solid contaminant may be delayed at the beginning of a cycle to allow a layer of oil, salt or moisture to build up first.

The spraying duration will depend upon the amount of accretion required to fully test the coating. Typically, the duration will be between 2 and 6 hours. After the predetermined time, the air supply is switched off and the sample is removed and re-weighed on a suitable weighing apparatus. From this, an assessment as to the level of deposition can be made by subtracting the pre-deposition weight from the post-deposition weight. As will be appreciated, the smaller the increase in weight, the betterforan anti-fouling coating.

Although not shown in Figure 2, it may be possible to use a general controller such as a work station or personal computer to interface with each of the controllable/adjustable parts of the apparatus. Hence, for example, a work station may be programmed to control the air pressure and flow rate, the feed rate of the first, second and third contaminants, and set the position of the sample through a desired cycle.

A second parameter which maybe relevant for assessing the suitability of a given surface finish, particularly in the aerospace industry, is the ease with which deposited contaminants can be removed during a cleaning process. Hence, in one embodiment, the method includes a cleaning procedure in which a soiled sample is placed in a cleaning cycle and re-assessed to determine how readily the deposits may be removed.

In one embodiment, the cleaning is achieved with an ultrasonic cycle in which the sample is placed in a warm water (approximately 40 degrees centigrade) ultrasonic bath and cleaned for 1 minute. After the cleaning, the sample is removed from the bath and dried before being re-weighed to ascertain how readily removable the deposit is.

Repeating this step multiple times can provide a more detailed insight into how easily the dirt can be removed. For instance, Figure 4 shows a plot of two samples, A and B, having different levels of contamination of an identical deposition process. The level of contamination is indicated by theY axis, and the number of cleaning cycles is provided by the X axis. Thus, it can be seen that after multiple 1 minute cleans in the warm water ultrasonic bath, the level of contamination on each sample is reduced.

It is also of note that the rate at which the samples are cleaned by the ultrasonic process varies, both between samples and between cycles for each sample. That is, the initial clean of sample B results in an approximate eighty percent reduction in the contamination.

However, subsequent cleans are less effective, removing comparatively less and less. The initial cleaning of sample A is not nearly so effective with only around thirty-five percent of the contamination being removed in the first cycle, but with substantial levels of removal in subsequent steps.

Thus, for an ideal anti-fouling surface would have the following attributes: * Small value C: indicating low mass of fouling accretion * Large value (steep gradient) of L: effective fouling removal in initial cleaning step * Small value of K: indicating little fouling retained at the end of the wash cycle.

A simple figure of merit can be calculated to compare and rank different anti-fouling surfaces, for example, C-4-K / L. In this case a low number indicates a better anti-fouling surface. With reference to Figure 4, the figure of merit clearly shows that sample A has better anti-fouling attributes than sample B.

I C 5 4 K 3 1 L 2 5

FOM = (D+E I G) 4 (worse) 1 (better) Other FOM values could be calculated orweightings altered according to properties of interest. The data analysis described is typical, but the data analysis and interpretation is not limited to the described process.

It will be appreciated that although the above described embodiments relate to coatings, this is not essential and the test method could be equally applicable to the surface finish of a non-coated article. Further, the test is equally applicable to other areas of industry such as window coatings, building cladding materials, power line insulators, automotive paint and railway rolling stock coatings.

Claims (15)

1. Claims: 1. A method of testing the antifouling properties of a sample surface, comprising: mounting a sample in a holder; mixing a contaminant with an air flow to provide contaminated air; supplying the contaminated air to a delivery device; projecting the contaminated air onto a surface of the sample using the delivery device; and, weighing the sample to ascertain the amount of deposited contaminant.
2. 2. A method as claimed in claim 1, wherein the relative position of the sample holder and delivery device is adjustable, and the method including the additional step of adjusting the angle of incidence at which the contaminated air is projected on to the sample.
3. 3. A method as claimed in either of claims 1 or 2, further comprising the step of mixing a first and second contaminant with the air supply to provide the contaminated air.
4. 4. A method as claimed in 3, wherein at least one of the first and second contaminant is a liquid, wherein the mixing of the liquid with the air flow includes introducing the liquid into the airstream via an atomiser.
5. 5. A method as claimed in any preceding claim including the steps of assessing providing a rate of deposition which is accelerated compared to the normal expected deposition rates for the component.
6. 6. A method as claimed in any preceding claim, further comprising cleaning the sample for a predetermined time and weighing the sample to establish how easily the sample can be cleaned.
7. 7. A method as claimed in any preceding claim, further comprising a steps of: analysing the weight data provided by weighing the samples pre-deposition, post-deposition and post-cleaning and, determining a fit for use factor.
8. 8. A method as claimed in any preceding claim, wherein at least one contaminant is a powdered material having a particle size below 10 microns in diameter.
9. 9. A method as claimed in any preceding claim, wherein the sample is representative of a gas washed surface in a gas turbine engine.
10. 10. An apparatus for testing the antifouling properties of a sample surface comprising: a sample holder for holding a sample to the tested; mixing apparatus for mixing an airflow with a contaminant so as to provide contaminated air; and, a delivery system for projecting the contaminated air on to the surface of the sample.
11. 11. An apparatus as claimed in claim 10, further comprising a plurality of mixing apparatuses.
12. 12. An apparatus as claimed in either of claim 10 or 11, wherein the delivery system includes an atomiser for creating an aerosol from a liquid contaminant and the air flow.
13. 13. An apparatus as claimed in any of claims 10 to 12, wherein the sample holder is adjustable such that the angle of incidence between the surface to be coated and the direction of the projected contaminated air can be adjusted to a predetermined angle.
14. 14. A system for testing the antifouling properties of a sample surface, comprising: the apparatus of any of claims 10 to 13 and a weighing apparatus.
15. 15. A system as claimed in claim 14, further comprising a cleaning apparatus for cleaning the surface of the sample.