, Description:A SYSTEM AND METHOD OF EVALUATING HIGH TEMPERATURE IMPACT BEHAVIOR OF COATINGS AND MATERIALS
FIELD OF INVENTION:
[001] The invention relates to the field of developing a method to evaluate high temperature impact behavior of surface coatings and materials used in applications involving impact damages.
BACKGROUND AND PRIOR ART AND PROBLEM IN PRIOR ART:
[002] Surface coatings are frequently subjected to impact and shock loads which deform the base material, thus placing stress on the adhesion and cohesion of the coating. For example, valve cone and valve seat are critical components in the steam turbine governing system that experience severe impacts at low temperature as well as at elevated temperature during these operations. These components are also prone to wear and self-welding damages due to high contact pressure and temperature. These require surface treatment or protective coating with adequate wear and impact resistance at designed temperature and pressure. Various coatings, such as chromium carbide coating by High Velocity Oxy Fuel (HVOF) process, NiCr coating, overlay by hard facing material, such as Stellite, Ultimet, Tribaloy or tungsten carbide applied by welding, spraying, HVOF or laser are being used for these components to protect them from the operational damages. The wear and high temperature erosion behaviour of these materials and coatings are often known and well established, however, their impact behaviour especially at high temperature is mostly unknown.
[003] According to US patent US3426578A, a method of investigating strength properties of solid materials by application of mechanical stress is explained by applying a single impulsive force generated only by free-falling weight. This invention relates to an impact testing apparatus for conducting controlled tests wherein a drop table releases a test specimen supported at a fixed angle for impact from a height above an impact base such that substantially no variation of the fixed angle for impact occurs. Considerable effort has been given to the development of testing machines which subject test specimens to controlled impact conditions in a test laboratory in order to perfect and prove the ability of the test specimen to perform successfully in actual operation. Development of a versatile impact testing machine is complicated by the fact that the required test conditions cover a wide range of impact parameters. Some of these impact parameters are duration, magnitude, and angle of the impact.
[004] US6523391B1 discloses a vertical impact testing apparatus comprising a rigid frame, an anvil connected to the frame, a dart positioned above the anvil, and a dropped-weight mechanism slidably connected to the frame. The frame is comprised of a vertical column, a base, and a guide rail. The vertical column extends for a sufficient distance to securely support a weight assembly through travel from various heights for testing a specimen. To facilitate downward travel of the dropped-weight mechanism at a proper vertical orientation, a guide block is rigidly attached to the weight assembly and the dropped-weight mechanism is slidably mounted to the guide rail. The dart is stabilized by a braced support arm connected to the frame and a bearing acting cooperatively to ensure impact of the dart with the specimen in the vertical plane and without tilt. The anvil is a solid structure, fixed in position, which contacts and supports the specimen on a side opposite of the dart as the dart impacts the substrate and forces it downward.
[005] US3412598A discloses a method of investigating strength properties of solid materials by application of mechanical stress by applying a single impulsive force, e.g. by falling weight generated by a compressed or tensile-stressed spring; generated by pneumatic or hydraulic means. A pair of bow limbs are positioned on either side of a track and a bow line attached to the outer ends of the limbs is attached to a carriage which rides on the track for carrying a test package. A source of power draws the bow, line against the bias of the bow limbs and a quick release mechanism releases the power source from the bow so that the carriage and package are propelled at high velocity toward an impact receiving surface. A stop halts movement of the carriage short of the impact receiving surface and the package continues in free flight to impact the surface.
[006] US2139527A Patent relates to impact-testing mechanism and more especially to a novel and useful mechanism for performing both Izod and Charpy impact tests. The invention consists in the novel parts, constructions, arrangements, combinations and improvements. A few industrial impact testers are available for determination of impact resistance, deformability and tensibility of coatings and substrates as well as adhesion of the coatings at room temperature. ERICHSEN Impact Tester 304, Elcometer 1615 Variable Impact Tester, TQC Impact tester SP1880 etc. are used to simulate ASTM D2794 test under standardised conditions. A ball punch of defined weight, the bottom of which is of fixed diameter, is dropped freely down a guide tube from an agreed or variable height. After the impact, the deformed zone of the specimen surface is examined for cracks and flaking. However, all these testers are designed to carry out impact testing at room temperature and high temperature testing is not available.
OBJECTS OF THE INVENTION:
[007] It is, therefore, an object of the invention to propose a method to evaluate high temperature impact behavior of surface coatings and materials used in applications involving impact damages.
SUMMARY OF THE INVENTION:
[008] Solution to one or more drawbacks of existing impact testing mechanism and additional advantages are provided through the present impact testing system and method as disclosed in the present disclosure. Additional features and advantages are realized through the technicalities of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered to be a part of the claimed disclosure.
[009] According to the invention, there is provided a system and method to evaluate high temperature impact behavior of surface coatings and materials. The invention consists in the novel design, constructions, arrangements and process. The high temperature impact testing is provided by repeatedly applying a single impulsive force, through pneumatically controlled drop of a specified weight ball from a specified height over a test specimen placed inside a controlled high temperature chamber. The invention also explains the method of carrying out repeated impacts over the test surface. The damage evaluation of coating/cladding/base material is carried out by visual inspections of the impacted surface as well as analysis of surface under optical microscope at various magnifications. The impact behavior of the coating/material is evaluated by its ability to withstand number of such impacts at a specified level of impact energy before failure in the form of crack, chip-off, rupture or delamination observed on the impacted surface.
[0010] The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] It is to be noted, however, that the appended drawings illustrate only typical embodiments of the present subject matter and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments. The detailed description is described with reference to the accompanying figures. In the figures, a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the figures to reference like features and components. Some embodiments of system or methods or structure in accordance with embodiments of the present subject matter are now described, by way of example, and with reference to the accompanying figures, in which:
[0012] Figure 1: Schematic of high temperature Impact tester.
[0013] Figure 2: Automatic ball releasing mechanism
[0014] Figure 3: High temperature specimen chamber
[0015] Figure 4: Room temperature impact testing in progress a) Ball ready to drop b) ball after release
[0016] Figure 5: High temperature impact testing in progress
[0017] Figure 6: Single Impact indents on different materials @5J impact energy
[0018] Figure 7: Impact damages at various test conditions
[0019] Figure 8: Impact damages of coating and cladding at different impact test conditions
[0020] Table 1: Impact Energy Range
[0021] The figures depict embodiments of the present subject matter for the purposes of illustration only. A person skilled in the art will easily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein.
DESCRIPTION OF THE PREFERRED EMBODIMENTS:
[0022] While the embodiments of the disclosure are subject to various modifications and alternative forms, specific embodiment thereof have been shown by way of example in the figures and will be described below. It should be understood, however, that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the scope of the disclosure.
[0023] The terms “comprises”, “comprising”, or any other variations thereof used in the disclosure, are intended to cover a non-exclusive inclusion, such that a device, system, assembly that comprises a list of components does not include only those components but may include other components not expressly listed or inherent to such system, or assembly, or device. In other words, one or more elements in a system or device proceeded by “comprises… a” does not, without more constraints, preclude the existence of other elements or additional elements in the system or device.
[0024] According to the invention, firstly, the high temperature impact tester is designed to conduct tests to determine the impact behavior of coated materials at different temperature by impact of steel balls. The instrument (Fig 1) consists of a sliding plate which can be moved to the required height by giving command from the controller referred as 116 in Fig.1; the sliding plate holds the automatic ball releasing mechanism. A scale is fixed vertically on the front panel of the instrument to see the sliding plate height from the bottom specimen. The bottom specimen whose impact behavior to be checked is fixed on the holder on the base plate of the instrument and is heated by an induction heater. The specimen and holder are covered properly to avoid any heat loss and the chamber has the provision of water cooling.
Specifications of High temperature Impact tester
Following are the broad specifications of the Impact tester:
Impact Ball Size: ½”, 1”, 1½”, 2”, and 2½”.dia, Material Steel
Specimen Size : 50x50mm & 100x100mm
Impact Height: Variable between 0.5-1.5 m with graduation every 50 mm.
Test Temperature: Variable between RT-800°C with digital display and control.
Temperature Sensor: K-Type thermocouple of 1200°C range
Air Supply: Compressed air @ 6kgf/cm2 pressure
Impact Energy: up to 15 Joules. (Table 1 show the impact energy range of the tester.)
[0025] Referring to Fig.1, the present invention provides description:
[0026] Constructional feature of High temperature Impact tester: The tester consists of the following assemblies: Vertical column assembly; sliding plate assembly, lift assembly and ball releasing assembly. All assemblies are mounted on a base plate over structure. The structure is made of steel with a base plate fixed on it. The base plate carries all tester assemblies.
[0027] Vertical column assembly referred as 106 in Fig.1: Four vertical columns are fixed on the base plate. The sliding plate slides on the guideways provided on the columns. A scale is fixed vertically on base plate and pointer is fixed on sliding plate, as sliding plate moves up & down the pointer indicated the height of ball above specimen.
[0028] Sliding Plate Assembly referred as 110 in Fig.1: A horizontal sliding plate consists of the guide ways for the balls to enable them for free movement up to ball releasing mechanism. The arrangements are made in such a way to accommodate 20 spherical steel balls having diameter of ½”, 1”, 1 ½”, 2”, 2 ½” which will be loaded on the guide ways.
[0029] Lifting Assembly referred as 104 in Fig.1: A wire rope arrangement is used to lift the sliding plate. It consists of drum having grooves formed on it to roll wire rope. One end of the wire rope is firmly tightened to drum and other end is tightened on to sliding plate after passing through two pulleys fixed on to plate. The drum is rotated by a motor and as it rotates, the wire roll over the drum lifting the sliding plate. Another two ropes are fixed onto the sliding plate to ensure its smooth travelling and the other end is connected to a counter weight for balancing.
[0030] Ball Releasing System referred as 108 in Fig.1: Fig 2 shows ball releasing mechanism fixed on the sliding unit. It picks the balls one by one and drop to the bottom specimen and is controlled by pneumatic system. It has a suction cup of different size to accommodate respective sized balls. Compressed air is directed to the vacuum generator through the release valve, which creates sufficient vacuum in the suction cup to hold the ball. Once the vacuum is released the ball drops. A sensor is provided to count the number of balls dropped. A ball guiding tube having end section made of Inconel alloy is provided from ball releasing point to the samples to restrict any kind of deflection of balls during free fall.
[0031] High temperature chamber referred as 114 in Fig.1: High temperature chamber is an enclosed chamber having specimen mounting platform as shown in fig 3. An induction heater is fixed on the base frame which heats the specimen to be tested. Induction heater and associated parts are water cooled through a recirculating water cooling unit referred as 118 in Fig.1. A mechanism is provided on the base plate which drives away the hit balls to the ball tray fixed on the base plate. Temperature feedback for controlling and maintaining the temperature is taken from the specimen holder as the specimen is subjected to impact and taking feedback from the specimen at testing time is not possible. This creates a difference in temperature of specimen and the set temperature. 10-70? difference is observed for nickel based super alloy specimen between the set temperature and actual temperature of specimen.
[0032] High temperature impact testing: As a part of experimentation, the chromium carbide (Cr3C2-NiCr) coating by HVOF process, stellite 21 cladding by conventional cladding and IN625 by laser cladding was carried out over IN617 samples. These sample were characterized for its hardness, microstructure and thickness. Impact testing of these coating at room temperature and high temperature were carried out using Impact tester. Fig 4 shows the impact test being carried out at room temperature. Since the test was conducted at room temperature, the top cover of specimen chamber was removed for better visual observations. Initially 20 nos of ball of specified size were loaded on the sliding plate. The sliding plate was moved to the required height to carry out impacts at particular impact energy value as per table1. The balls were dropped one by one with ball release mechanism programmed through controller referred as 106 in Fig.1. In fig 4 step wise movement of the ball is visible from ball drop point to the specimen.

[0033] Fig 5 show the high temperature impact testing being carried out on the sample. For high temperature initially the sample is heated upto required temperature. After the temperature gets stabilised, the balls were impacted on the sample as per the set program. After each 20 impacts the damage was examined through image analyser for detecting any damage. The visual examination of the damage is also done to see any delamination of coating in between the impacts.

[0034] Testing Evaluation: At the start of the impact test, single impact indent has been analyzed on all the three types coating i.e a) Chromium carbide HVOF coating, b) Stellite 21 cladding and c) alloy 625 cladding at room temperature. These impacts were made at an impact energy of 5J using 2” dia ball dropped from a height of 1 m. Figure 6 shows single impact indents at 12.5x magnification using image analyzer on these materials respectively. It is evident from the indents that indent sizes are increasing with the decreasing in hardness of the coating. The indent is smallest (~5mm) for hard chromium carbide coating and largest for alloy 625 cladding (~7.5mm). This is due to the fact that at the initial moment of impact the contact between ball and coated surface is point contact but as the ball travels further due to the impact force, the material tends to deform. This deformation is more for softer material resulting in bigger impression of the ball. It is also observed that no cracks were observed at the middle of the indent however in HVOF coating the coatings were ruptured at the edges, however, no delamination of coating was observed. The edges in both the cladding are visually unaffected due to impact.
[0035] Fig 6 illustrates the single impact indents on different materials @5J impact energy.
[0036] In the next set of tests, coatings/materials were tested with multiple impacts at different temperatures. Impact energy levels were changed by changing ball size and drop height as per table 1. The indents were analyzed under image analyzer at 100x magnification to observed any kind of cracks, delamination or failure of coating. Figure 7 shows different kind of damages occurred at various conditions. For the materials that are tested for the first time, initially test parameters were established on a trial sample at room temperature. This sample was tested for impacts at various impact levels at room temperature to decide about the impact level such that the significant damage can be seen on the samples. After initial trials the impact level can be fixed and all the further test can be conducted at this impact level. Number of impacts were recorded at the initiation of the first crack and at subsequent failure of coating. Number of impacts for each set of testing can be decided depending upon nature of the material. The numbers of impacts can be less for brittle material and more for ductile materials. It was observed that at higher temperature the impact resistance of both coating and cladding is better as compare to room temperature.
[0037] Fig 8 shows the difference in physical impact damages of coating and cladding at different impact test conditions.
[0038] Fig 8: Impact damages of coating and cladding at different impact test conditions as shown in table 1 below.
Table 1: Impact Energy Range
Ball size (Dia) Ball weight Drop distance Impact energy
2 ½”
(63.50 mm) 1.04
kg
0.5 m
1.0 m
1.5 m 5.1 J
10.2 J
15.3 J
2”
(50.80 mm) 0.530
kg 0.5 m
1.0 m
1.5 m 2.6 J
5.2 J
7.8 J
1 ½”
(38.10 mm) 0.225
kg 0.5 m
1.0 m
1.5 m 1.1 J
2.2 J
3.3 J
1”
(25.40 mm) 0.065
kg 0.5 m
1.0 m
1.5 m 0.32 J
0.64 J
0.96 J
½”
(12.70 mm) 0.008
kg 0.5 m
1.0 m
1.5 m 0.04 J
0.08 J
0.12 J

[0039] In an embodiment of the invention, the present invention provides following steps of testing method for evaluating high temperature impact behavior of surface coatings, cladding, thin films and substrate materials comprising of:
1. Mounting of sample in the high temperature impact tester
2. Selection of ball size and drop height based on impact energy level as mentioned in Table 1.
3. Positioning the ball mounting assembly at the specified height.
4. Loading the 20 numbers of balls of selected size.
5. Mounting of appropriate suction cup corresponding to ball size.
6. Setting the selected test temperature and switching on the heater and cooling unit for high temperature testing.
7. Mounting the guide tubes from ball releasing point to high temperature chamber.
8. Waiting for the temperature to stabilize upto set value.
9. Starting the test for automatic impact of ball for specified number of impacts.
10. Switch off the heater and wait to reach the temperature till ambient.
11. Switch off the cooling unit.
12. Removal of the sample for visual/optical microscopic analysis.
13. Repeat the test till failure of coating/material.
[0040] It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.”
[0041] It will be further appreciated that functions or structures of a plurality of components or steps may be combined into a single component or step, or the functions or structures of one-step or component may be split among plural steps or components. The present invention contemplates all of these combinations. Unless stated otherwise, dimensions and geometries of the various structures depicted herein are not intended to be restrictive of the invention, and other dimensions or geometries are possible. In addition, while a feature of the present invention may have been described in the context of only one of the illustrated embodiments, such feature may be combined with one or more other features of other embodiments, for any given application. It will also be appreciated from the above that the fabrication of the unique structures herein and the operation thereof also constitute methods in accordance with the present invention. The present invention also encompasses intermediate and end products resulting from the practice of the methods herein. The use of “comprising” or “including” also contemplates embodiments that “consist essentially of” or “consist of” the recited feature.

Claims:We Claim:
1. A system to evaluate high temperature impact behaviour of various surface coatings, cladding, thin films and substrate materials within a housing comprising:
a vertical assembly with a plurality of vertical columns mounted over a base plate present at the lowest part of said housing;
an adjustable sliding plate coupled to a plurality of guideway present in the plurality of vertical columns;
a lifting assembly connected to the sliding plate consists of a drum(s) having grooves to roll a wire rope connected to the sliding plate at one end and to the drum(s) at other end;
a ball releasing assembly with a suction cup at proximal end fixed to the sliding plate, the ball releasing assembly consists of a ball guiding tube with a proximal opening towards the ball releasing assembly;
a high temperature chamber having a specimen mounting platform and a specimen; the high temperature chamber connected to distal opening of the ball guiding tube; consists of an induction heater fixed at the base plate.
2. The system as claimed in claim 1, wherein the vertical assembly consists of four vertical columns, a scale is fixed vertically on the base plate with a marking pointer fixed on sliding plate indicative of the height of ball above the high temperature chamber as sliding plate moves up and down.
3. The system as claimed in claim 1, wherein the sliding plate is horizontally placed on the guideways of vertical columns consisting of guide way contours enabling the transfer of a ball in a guided manner to the ball releasing assembly.
4. The system as claimed in claim 1, wherein the sliding plate is capable of accommodating 20 balls of multiple size and dimension.
5. The system as claimed in claim 1, wherein the lifting assembly consists of a plurality of pulleys providing channel to the wire ropes for movement.
6. The system as claimed in claim 1, wherein the lifting assembly consists of a motor connected to the drum(s), powering the drum(s) rotation leading to the rolling of the wire rope on the drum lifting the sliding plate.
7. The system as claimed in claim 1, wherein the lifting assembly consists of multiple wire ropes connected to the sliding plate at one end and to a counter weight on the other end.
8. The system as claimed in claim 1, wherein the ball releasing assembly consists of the suction cup capable of holding the balls from the sliding plate and releasing the ball in the ball guiding tube through the proximal opening using pneumatic pressure.
9. The system as claimed in claim 1, wherein the suction cup uses compressed air and vacuum to hold the ball.
10. The system as claimed in claim 1, wherein the ball releasing assembly consists of a sensor to count the number of balls released.
11. The system as claimed in claim 1, wherein the ball releasing assembly is controlled by a controller.
12. The system as claimed in claim 1, wherein the ball releasing assembly consists of a ball guiding tube having end section made of Inconel alloy.
13. The system as claimed in claim 1, wherein the specimen present in the high temperature chamber receives impact of the ball at specified speed through the distal opening of the ball guiding tube.
14. The system as claimed in claim 1, wherein the induction heater present in high temperature chamber provides heat to the specimen.
15. The system as claimed in claim 1, wherein the high temperature chamber consists of a temperature feedback mechanism connected to a specimen holder.
16. The system as claimed in claim 1, wherein the high temperature chamber is connected to a water cooling unit.
17. The system as claimed in claim 1, wherein the base plate consists of mechanism to drive away the balls to a ball tray fixed on the base plate.
18. The system as claimed in claim 1, allows removal of the specimen from the high temperature chamber for visual or optical analysis.
19. A method to evaluate high temperature impact behaviour of various surface coatings, cladding, thin films and substrate materials comprising:
mounting a specimen in a high temperature chamber;
heating the specimen in the high temperature chamber;
positioning a ball mounting assembling at a specific height above the high temperature chamber;
mounting a suction cup corresponding to the ball size in the ball mounting assembly;
loading a plurality of balls in a sliding plate connected with the ball mounting assembly;
picking the ball with the suction cup and releasing the ball in the high temperature chamber providing a single impact force on the specimen;
analysing the specimen for the impacts.
20. The method as claimed in claim 19 wherein the single impact force is applied in a repeated form.
21. The method as claimed in claim 19 wherein the release of balls is controlled using pneumatic force.
22. The method as claimed in claim 19 wherein the high temperature chamber provides variable temperature up to 800 degrees Celsius.
23. The method as claimed in claim 19 wherein the balls released and received in the high temperature chamber are delivered to a ball tray fixed at the base plate.
24. The method as claimed in claim 19 wherein the ball is released in the high temperature chamber through a ball guiding tube.
25. The method as claimed in claim 19 wherein the damage evaluation of coatings/materials is carried out by visual inspections of the impacted surface as well as analysis of surface under optical microscope at various magnifications. The impact behaviour of the coating/material is evaluated by its ability to withstand number of such impacts at a specified level of impact energy before failure in the form of crack, chip-off, rupture or delamination observed on the impacted surface.