Surface testing apparatus

Abstract

A method for performing material testing comprises: a) causing a test probe 92 to impact a test surface of a sample 10 at a test site to cause impact damage in a region surrounding the test site, b) r

Description

24 07 23 5 This invention relates to surface testing apparatus. In particular, it relates to apparatus for performing simulation of erosion testing. In traditional laboratory solid-particle erosion tests, such as ASTM G76, solid particles are accelerated to hit a surface in a high-velocity gas stream. The particle velocity is difficult to measure accurately, such measurement requiring advanced techniques such as laser Doppler 10 velocimetry or particle image velocimetry. The surface is eroded over a region of several mm2 and average erosion rates are determined ex-situ from mass loss or with in-situ, non-contact approaches such as laser scanning for real-time volume measurements. Nano-impact testing has been developed to operate by studying impact on a much more localised scale than erosion testing procedures. An example is disclosed in GB-A-2 475 683 of 15 the present applicant In such techniques, each impacteventis a high strain-rate contact, which causes multiple single-impact damage rather than the average erosion rates determined from conventional erosion tests. In nano-impact testing, single impacts and repetitive impacts at the same position are possible. Arrays of repetitive impact tests can be set up to study influences of (i) cumulative impact energy and (ii) statistical variation. 20 The approach described above provides a significant increase in sensitivity as compared to macro-scale erosion testing. However, it is it is not possible to follow the real progression of impact or erosive wear processes where material is removed by way of cracking and wear from impacts occurring not at the same location but close enough so that the damage from one impact influences subsequent behaviour. 25 An aim of this invention is to provide a system for performing surface testing that provides similar information to solid-particle erosion tests while avoiding the complication of existing testing systems. 24 07 23 To this end, from a first aspect, this invention provides a method for performing material testing comprising: a. causing a test probe to impact a test surface of a sample at a test site to cause impact damage in a region surrounding the test site; 5 b. repeating step i. multiple times at different test sites; and c. performing an analysis on the accumulated damage after completion of steps a. and b. Embodiments of this invention can simulate real erosion processes by creating randomly-spaced impacts in a set region. The location of the test sites can be controlled in various ways, including, but not limited to: 10      • random rectangular distribution (that is, there is an equal probability of next impact occurring anywhere within the set region); • normally-distributed around a centre point (that is, there is a greater probability near to the centre, which may reflect what happens in many erosion situations better such as in gas-jet erosion tests); or 15      • at a given set distance away from each other (to statistically study the influence of pile- up / cracking from adjacent impacts). Each impact event is a high strain rate contact. Preferably, one or more of the impact energy, probe geometry, size of the region and the number of impacts are adjustable. Computer control for multiple arrays of experiments can be configured on the same test 20 samples. The greater control in the nano-scale tests allows erosion mechanisms used in embodiments of the invention, in particular the start of damage processes, to be determined in much greater detail than is possible in macro-erosion tests or in the known nano-impact systems. 24 07 23 The test probe typically impacts the test site at 90° to the test surface. In alternative embodiments, the test probe impacts the test surface at an angle other than 90° and step a. further includes making a determination, experimentally, by calculation or otherwise, of the position at which the impact will occur prior to causing the test probe to impact the sample. 5 Step a. may include the probe making multiple impacts with the test surface at the same test site. For example, this may occur as a result of the probe rebounding from the test surface. In embodiments of the invention, the sample may be subject to environmental control of one or both of temperature and humidity. In further embodiments, the sample may be immersed in a liquid. 10 Analysis methods may include determination of how the impact depth and / or dynamic hardness vary with each impact For example, the methods may include study of the influence of work-hardening and / or cracking on the progression of erosive wear with the number of impacts within the impacted area. This analysis can be combined with other methods of assessing wear volume such as confocal 15 microscopy of the eroded regions. From a second aspect, the present invention provides material testing apparatus comprising: a sample stage for carrying a sample of material to be tested; an impact assembly including a test probe, the impact assembly being operable to cause the test probe to impact a sample carried on the sample stage so as to cause impact damage to 20 the sample; and a drive assembly operable to move the sample stage and the impact assembly with respect to one another whereby the position on the sample at which impact damage is caused can be varied; and a controller programmable to cause the drive assembly to vary the relative positions of 25 the sample and the impact assembly such that impacts can be performed on a sample in a predetermined arrangement during a testing sequence. Such apparatus can be used to perform testing by causing multiple distributed impacts for example to perform a method embodying the first aspect of the invention. Typically, the impact assembly includes a probe that makes contact with a sample during an impact event, the impact assembly includes a probe that makes contact with a sample during an impact event The probe may be driven into the sample electromagnetically during an impact event. The impact assembly may includes a pendulum rod on which the probe is carried. Embodiments of the invention may further include heating or cooling apparatus operable to control the temperature of a sample being tested and / or a liquid cell within which a sample being tested can be immersed in a liquid. An embodiment of the invention will now be described in detail, by way of example, and with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic illustration of apparatus embodying the invention; Figure 2 shows a programmed pattern of impacts to be generated by an embodiment of the invention within a test region of a surface; and Figure 3 shows impacts in borosilicate glass during testing in apparatus embodying the invention; and Figure 4 shows impacts in borosilicate glass following testing in apparatus embodying the invention. With reference to the drawings, apparatus embodying the invention is shown in Figure 1 as it will be disposed for use. The terms "upper”, "lower” and related terms should be interpreted as referring to the apparatus in this position. What will be referred to as the "front” of the apparatus is towards the left of Figure 2, and the "rear” is towards the right. The terms "forward” and "rearward” should be interpreted accordingly. The up and down direction will be deemed to be the Z-axis; into and out of the figure, the Y-axis; and left to right, the X-axis. Test apparatus embodying the invention comprises two main assemblies: the sample-holding assembly and the probe assembly. With reference to Figure 1, the sample-holding assembly comprises a sample holder 12 and other components that act as a drive system for the sample holder 10. The sample holder 12 operates to hold a sample 10 that is to be subject to testing. The sample holder is carried on a stage that can be driven to move the sample holder along the X, Y and Z axes. The probe assembly will now be described. Although not shown in the figures, the probe assembly is constructed on a rigid base that is essentially immovable with respect to the stage 16. Components that are described as "fixed” should be taken as being immovable with respect to the rigid base. The probe assembly comprises a vertical pendulum rod 80 formed from ceramic such as high-density alumina. The rod 80 is carried on a fixed pivot 82 that allows the rod 80 to pivot freely about a horizontal axis that is transverse to a long axis of the sample holder 12 (which corresponds to the X-axis). A coil 84 is carried on the pendulum rod 80 above the pivot 82, the coil 84 being positioned next to a fixed magnet 86. Variation of current in the coil 84 can cause it to be attracted towards or repelled from the magnet 86, with a variable force, so urging the rod to move about the pivot 82. An adjustable limit stop 88 is used to ensure that the rod 80 adopts a vertical disposition to whereby that the axis of the probe 92 (see below) is normal to (or is ata specific desired angle to) the specimen 10. A soft iron block 70 is carried on the rod 80. A solenoid 72 is secured to the base adjacent to the soft iron block 70. Also carried on the rod 80 is an adjustable balance weight 98 that can be moved along a screw thread to vary its position with respect to the rod 80. This is adjusted to ensure that the rod 80 is balanced in a near-vertical position immediately before testing commences (that is, once the probe assembly is complete). Adjustment of current in the coil 84, and consequent variation in the force between it and the magnet 86, can be used to reduce or eliminate any remaining deviation of the rod 80 from vertical. A probe holder 90 is carried on the rod 80 below the pivot 82. The probe holder 90 has a horizontally projecting rod that projects towards and is substantially coaxial with the sample holder 12. A probe 92, which serves as the test target, is carried on an end surface of the horizontally projecting rod and is directed towards the sample 10. The probe holder 90 carries a conductive plate 94 that faces directly away from the probe 92 on the opposite side of the rod 90. The conductive plate 94 is closely spaced from and parallel to a fixed plate 96. The two plates 94,96 together form a capacitor, the capacitance of which varies as the spacing between the plates 94,96 changes as a result of pivotal movement of the rod 80. As is well recognised, it is possible to measure the capacitance of a variable capacitor and, with suitable calibration, establish a relationship between the capacitance and the separation of the plates 94, 96. After calibration, this allows the instantaneous position of the probe holder 90, and therefore, the probe 92, to be measured. A damper 78 comprises a pair of optically flat damping plates with variable spacing. One plate is fixed and the other is attached to the rod 80. Normally, during testing, the spacing is set to be large and air damping that occurs between the plates is insignificant. The spacing can be reduced to reduce the impact velocity during testing, if required. The damping plates are normally used to alleviate the effects of vibration in high sensitivity indentation testing. Operation and use of the apparatus will now be described. To use the apparatus, a sample is mounted on the sample holder and a suitable probe is selected. Test probes may be similar to those used in conventional test apparatus, these being multi-faceted (e.g., pyramidal) or spherical diamonds. However, for impacting softer materials the probe material could, in the alternative, be a ceramic or a hard metal. Pyramidal-shaped probes may be 3-sided, 4-sided or conical, with a tip radius of the order of 100 - 500nm. A preferred pyramid shape is the cube corner. Spherical probes can range in diameter from 2 pm to several mm. A current is then applied to the coil 84 of just sufficient magnitude to cause the rod 80 to rotate about the pivot 82 until the rod is vertical, to set the default position of the test target. This compensates for any variation in the weight of the probe 92 and normalises the output from the capacitor 94, 96. To perform a single impact between the probe 92 and the sample 10, the following steps are followed: 1. Initially there is a gap between the solenoid 72 and the soft iron 70, and the rod 80 is held against the limit stop 88 by maintaining a small current in the coil 84. 2. The sample 10 is moved towards the test probe 92 until contact is detected via the parallel plate capacitor 94, 96. 3. The solenoid 72 is energised such that the rod 80 is pulled back until the solenoid 72 and soft iron 70 are in contact. 4. The sample 10 is moved inwards a pre-defined distance. 5. The X position of the sample 10 (specifically, the surface plane to be tested) is now fixed. 6. A pre-calibrated static "impact” current is applied to the coil 84 with the solenoid 72 still preventing the rod 80 from moving. 7. Current in the solenoid 72 is switched off to allow the force at the coil 84 to rotate the rod 80 around the pivot 82 and thereby accelerate the test probe 92 into contact with the sample 10. The position v. time of the test probe 92 is monitored throughout via the capacitor 94, 96. 8. For the next impact, the solenoid is again energised, the sample is moved to another position in the YZ plane, and the process is repeated. Movement at step 8 may follow a predetermined path, programmed in the controller, or may be a random movement within a test area of the sample. After producing an impact at step 7 above, the current in the coil 84 is typically maintained. Recoil occurring after a first contact is then followed by further impacts from ever-decreasing impact distances, caused by the probe 92 bouncing on the sample 10. Alternatively, bouncing can be precluded by using software control to re-energise the solenoid 72 immediately after the first contact, thus preventing subsequent rotation of the rod 80 until a further impact is initiated following step 8. The following describe variations modifications that may be used individually or in combination in embodiments of the invention. 1. The damper 78 uses either eddy current damping or squeeze-film air damping, and may be used in certain cases to produce very low velocity impacts. Damping limits the maximum velocity that can be attained as the probe 72 accelerates towards the sample 10. 2. Any impact apparatus may be used instead of the assembly described above. For instance the impact apparatus of GB-A-2 569 442 (the content of which is incorporated herein by reference) may be used. 3. Instead of moving the test probe 92 as described, the probe may be held in position under a static force, (e.g. against the limit stop 88), and the sample 10 may be moved towards the probe 92 to produce an impact This may, for example, be done using apparatus disclosed in GB-A-2 475 683. Again, after each impact the sample 10 is moved in the YZ plane to another random or programmed position by means of the sample manipulator stage. 4. High speed sample movement may employ a stage with piezoelectric actuators for XYZ movement With this arrangement, random oscillations may be applied in each direction, including X, such that the test probe produces rapid sequential impacts to form an impact wear crater. Figure 2 shows a typical pattern of locations 74 within a test area of a sample 10. It will be seen that these may either be spaced apart on the surface or overlapping. Figure 3 shows a sample under test part-way through a test sequence. This shows a piece of borosilicate glass that has been subject to multiple impacts randomly distributed over a surface area. Each impact produces an approximately circular central impact indentation from which a one or more radial crack extends. It will be seen that one or more radial crack from one an impact may extend to intersect with a radial crack or central impact of another impact Many additional impacts are performed to complete a test sequence which results in a region of damage within the test area which results in an approximation of the effect of erosion on the test area. Such a region of damage at the end of a test procedure is shown in Figure 4. The test procedure described above relates to testing a flat sample with an impact direction of 90° to the test surface. In certain cases, however, it is advantageous to impact a surface at a different angle, for instance when simulating the effect of erosion on surfaces subjected to a glancing particulate stream. To accomplish this, a sample is mounted at a chosen angle. 5 However, the position along the X-axis at which an impact occurs will vary as the sample is moved parallel to the Y and Z axes. The location of the impact point must be determined prior to each impact. This can be done experimentally prior to each impact or by calculation based on knowledge of the angle and the Y and Z positions. Erosion under different environmental conditions may be simulated by performing testing in 10 specific conditions. For example, testing may be performed at a particular humidity or with the sample 10 immersed in a liquid cell attached to the sample stage. In addition, heating or cooling apparatus may be placed directly beneath the sample for temperature control. Following completion of the test sequence, the test area can be inspected using conventional methods to deduce information about the properties of the sample under test

Claims

1. A method for performing material testing comprising:a. causing a test probe carried on a pendulum rod to impact a test surface of a sample 5              ata test site to cause impact damage in a region surrounding the test site;b. carrying out step a. at multiple different test sites within a test area of the test surface; andc. performing an analysis on the accumulated damage after completion of steps a. and b.30 08 2410      2. A method according to claim 1 in which the locations of test sites is varied inaccordance with a random rectangular distribution.

3. A method according to claim 1 or claim 2 in which the locations of test sites is varied around a normally-distributed centre point.

4. A method according to any preceding claim in which the locations of test sites are a set 15          distance away from each other.

5. A method according to any preceding claim in which each impact event is a high strain rate contact.

6. A method according to any preceding claim in which one or more of the impact energy, probe geometry, size of the test region and the number of impacts are adjustable.20      7. A method according to any preceding claim in which the test probe impacts the test siteat 90° to the test surface.

8. A method according to any one of claims 1 to 6 in which the test probe impacts the test surface at an angle other than 90° and step a. further includes making a determination30 08 24of the position at which the impact will occur prior to causing the test probe to impact the sample.

9. A method according to any preceding claim in which step a. includes the probe making multiple impacts with the test surface at the same test site.5      10. A method according to claim 9 in which the multiple impacts occur as a result of theprobe rebounding from the test surface.

11. A method according to any preceding claim in which the sample is subject to environmental control of one or both of temperature and humidity.

12. A method according to any preceding claim in which the sample is immersed in a liquid.10      13. A method according to any preceding claim that includes determination of how theimpact depth and / or dynamic hardness vary with each impact14. A method according to any preceding claim that includes study of the influence of workhardening and / or cracking on the progression of erosive wear with the number of impacts within the impacted area.15      15. A method according to any preceding claim that includes confocal microscopy of theeroded regions.

16. Material testing apparatus comprising:a. a sample stage for carrying a sample of material to be tested;b. an impact assembly including a test probe, wherein the impact assembly includes20             a pendulum rod on which the probe is carried, the impact assembly being operableto cause the test probe to impact a sample carried on the sample stage to cause impact damage to the sample; andc. a drive assembly operable to move the sample stage and the impact assembly with respect to one another whereby the position on the sample at which impact damage 25             is caused can be varied; andd. a controller programmable to cause the drive assembly to vary the relative positions of the sample and the impact assembly such that impacts can be performed on a sample in a predetermined arrangement during a testing sequence.

17. Material testing apparatus according to claim 16 in which the probe is driven into the 5          sample electromagnetically during an impact event18. Material testing apparatus according to claim 16 or claim 17 further including heating or cooling apparatus operable to control the temperature of a sample being tested.

19. Material testing apparatus according to any one of claims 16 to 18 further including a liquid cell within which a sample being tested can be immersed in a liquid.30 08 24

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