Rotating Sample

volume loss~Vr=40.43 Wr Dr (cubic centimeters) Wr and Dr both in inches

The volume loss calculations were made assuming the volume of scar on the stationary sample is an ellipsoid and that on the rotating sample is the volume traced by an ellipse when moving along the circumference.

Commercial crossed cylinder machines are available, although some machines have been custom designed and built.

Block-on-Ring Test

One of the most versatile tests for sliding wear of coatings is the block-on-ring test. A rotating metal ring is loaded against a fixed block, making a line contact when the test is first begun. The coatings are usually applied to the contact surface of the block, but in some cases, both the block and the ring are coated. The potential materials combinations are limitless. In addition, the test can be run with selected liquids, gases, or lubricants to simulate a variety of service conditions. Rotational speed and load can also be varied as needed. One may select a variety of test conditions, or use the ASTM G77-83 standard method (2) for measuring both sliding wear and coefficient of friction.

A schematic of the block-on-ring specimen configuration is shown in Figure 2. Wear is measured by calculating the volume loss of the block material through the scar volume equation shown in Figure 3. The volume loss of the ring is determined from a weight loss conversion to volume by dividing by the material density.

Some block-on-ring machines are custom made, but commercial versions are available.

Pin-On-Disc Test

There is probably more variation in design, specimen siz.e, and operating conditions in pin-on-disc testers than in any other type of wear test. Most pin-on-disc machines are used for measuring sliding wear and friction properties, but severe adhesive wear, or galling, is studied as well. For testing coatings, the coating is usually applied to the end of a flat-ended or hemispherical pin, although coated discs are used as well. A schematic of a typical pin-on-disc apparatus (3) is shown in Figure 4. Two modes of operation are shown; in the left diagram the pin presses against the same path on the disc for repeated revolutions;

however, the pin spirals into a continuously new path as the disc revolves in the figure on the right. This test is very versatile, since the testing conditions can be varied greatly. One can use a furnace to study material wear and friction properties at elevated temperatures and in various controlled atmospheres. Most pin-on-disc is "home-brew".

Abrasion - Low Stress Taber Test

The Taber Abraser is a commercial wear tester designed to measure the low stress abrasive wear resistance of a wide variety of materials and coatings. Low stress abrasive wear is defined as wear due to hard particles or hard protuberances forced against and moving along a solid surface where particle loading is insufficient to cause fracture of the hard particles or protuberances. A schematic of the Taber apparatus is shown in Figure 5. Abrasive wear of flat plate specimen is caused by the action of a pair of rubber bonded abrasive wheels that are loaded normal to the specimen. The specimen, usually a metal plate with a coating applied to the test surface, is rotated at 72 rpm by a turntable. The two abrasive wheels are caused to drag and rotate, thus abrading the coating. Various abrasive wheels with differing abrasive quality are available for testing a range of materials. Wear is determined by: weight-loss method if all materials have similar densities; volume loss method if the material densities differ (volume-loss equals weight-loss divided by density); depth of wear scar, measured with an optical micrometer.

The Taber Test is useful for evaluating the abrasive wear of coatings that can be applied to a flat plate. It has been particularly successful for evaluating electroplated and electroless coatings. A standard test method, ASTM D1044, is available for the Taber Tester. A modification to the Taber test allows introduction of loose abrasive particles.

Dry Sand Rubber Wheel Test

Another very popular low stress abrasion test is the Dry Sand Rubber Wheel Test (DSWR). This test is used to rank the abrasion or scratch resistance to silica sand of many classes of materials. The test is especially appropriate where dry abrasion conditions need to be simulated. A schematic of the test apparatus is shown in Figure 6. Abrasive wear on the test specimen is caused in a uniform manner by sand particles being trapped between the specimen and a rubber rimmed steel wheel and dragged along as the wheel rotates. A cantilevered load system presses the specimen against the rotating wheel with a constant force.

A standard practice, ASTM G65-81, provides four different procedures to allow testing of a wide variety of materials (4). In each procedure, a standardized abrasive is used; a rounded quartz, grain sand. The 50/70 mesh sand designated AFS 50/70, is supplied by a single source to provide uniformity. This supplier is Ottawa Silica Co., P.O. Box 577, Ottawa, IL 61350.

A wide variety of coating types, including ceramics, thin coatings, metal alloys, surface modifications, and hardface materials can be evaluated with this test.

Abrasion- High Stress

High stress abrasion occurs when the load is sufficient to fracture the abrasive particles. Tests for high stress, abrasion are similar to those for ,low stress abrasion, but the loading components are designed to cause fracture of the abrasive. Described below are two tests for high stress conditions, one performed wet and one dry.

Pin-on-Drum

A number of pin type tests are available for testing coatings. Some apply a. loaded pin against a rotating disc which is covered with an abrasive paper. Others have a loaded pin traveling linearly over an abrasive paper. A very effective method to accomplish this type of action was developed by Mutton at the Melbourne Research Laboratories in Australia (5). This method was improved upon by the Bureau of Mines (6), and their apparatus is shown in Figure 7. The equipment consists of a head that rotates the test pin while traversing the length of a rotating drum covered with abrasive paper. Typical abrasives used are alumina, silicon carbide, and garnet. The test is designed so that the rotating pin, upon which a coating has been applied to the test end, engages the abrasive paper in a spiral so that the pin always contacts fresh abrasive. The pin is loaded with sufficient weight to produce crushing of the abrasive particles to insure high stress abrasion conditions. Very hard and brittle coatings are sometimes subject to spalling on the edges due to high Hertzian loading at the unsupported edge.

Alumina Slurry Test

To simulate highly abrasive conditions in a liquid medium, the Alumina Slurry test was developed, and adopted by ASTM as standard procedure ASTM B611 (7). This test is similar to the Dry Sand Rubber Wheel Test, except that a steel wheel is rotated against the flat coated specimen in slurry containing sharp alumina particles. The similarities to the rubber wheel test can be seen in Figure 8.

While the alumina slurry test was originally developed for testing the abrasion resistance of cemented carbides, it has been used successfully on hard coatings, diffusion alloyed materials, ceramics, and hardfaced materials. As with the rubber wheel test, specimen preparation is uncomplicated. Rectangular specimens can be easily coated or surface processed for testing.

Abrasion-Gouging

Gouging wear is a very high stress form of abrasion which is observed in heavy equipment for mining and earth moving operations. It is characterized by high rates of material removal from components that engage rock, ore, and heavy earth formations. A mineral operation that produces gouging wear is jaw crushing of rock or ore. A jaw crusher test has been developed for measuring gouging wear quantitatively.

Jaw Crusher Test

Several researchers have been involved in developing and improving the jaw crusher test (8), (9). The jaw plates that engage the hard abrasive rock are the test specimens. Each jaw contains one plate of the test material and one plate of a reference material. After a given amount of a standard rock is crushed, the jaw plates are reevaluated for wear. Weight losses are converted to volume losses, and the ratio of the volume loss of the test plates to the volume loss of the referenced plates is calculated. This wear ratio is reported as the gouging abrasion resistance of the test material. An ASTM standard (10) has been written for the jaw crusher test, and a schematic for the equipment is shown in Figure 9. This test would be much too severe for most types of coatings, but is frequently used to evaluate extreme abrasion behavior on weld-overlay hardfacing.

Erosion-Dry Particle

Several styles of dry particle erosion testers have been developed for measuring the resistance of materials to a gas driven stream of erosive particles A correct term for this type of wear is solid impingement erosion. The extent of the erosion damage to a target material is closely related to such test variables as impingement angle (between particle flow direction and target surface at point of impact), particle velocity (related to velocity to the power 2 to 4+), particle hardness, particle shape and size, hardness and microstructure of target, and temperature. Numerous erosion testers have been developed, but the most popular seems to be the jet impingement erosion tester, and that will be described here.

Jet Impingement Test

Many of the jet impingement erosion testers are designed to provide test conditions recommended in the ASTM Standard practice G76-83 (11). A schematic of this equipment is shown in Figure 10. It is important to provide a particle feed system that delivers a carefully controlled feed rate of abrasive to the mixer and nozzle. Considerable variation in test results will occur if feed rate fluctuates. Test results will also be unreliable if the gas pressure varies during a test, or from test to test. The particle velocity, controlled by gas pressure, is the most heavily weighted test variable in an erosion test. This is especially true for hard, brittle materials, where the wear can vary by as high as the 4.4 power of particle velocity (12).

Most researchers report the erosion behavior of a material at one or two impingement angles for a single particle velocity and particle type. That much information is at times sufficient to rank a number of materials at those angles. However, in order to completely characterize the erosion behavior of a material, it is important to test the material at several particle impingement angles, at several particle velocities, and with two or three abrasive materials with a range of particle characteristics (hardness, size, shape).

An example of a complete erosion study (13) on a material with a single abrasive type is shown in Figures 11, 12, and 13. The erosion wear data are plotted as a function of six particle impingement angles in Figure 11. The four curves represent the erosion/angle behavior at four different particle velocities. The same erosion wear data from the 24 tests (6 angles x 4 velocities) can be plotted as in Figure 12 where erosion wear is shown as a function of four particle velocities. These six curves show the erosion/ velocity behavior at six different impingement angles. The accumulation of data from the 24 erosion tests can thus be graphically presented to allow design engineers a way to easily select material with certain specific erosion properties. It is obvious that improper selections of materials could be made if data from only one or two tests were considered. It is also worthwhile to present a three-dimension plot showing erosion wear as functions of both impingement angle and particle velocity, as shown in Figure 13. On this type of graphical presentation, the results of the interaction of angle and velocity on wear can be easily evaluated.

The production of 24 or more erosion tests to completely characterize the erosion behavior of a material is very time consuming and expensive, especially if repeated for several types of abrasive (48 tests for two abrasives, 72 tests for three, etc.) Yet, a wear data base of engineering.materials and coatings is badly needed to aid in proper selection and utilization of many materials. In answer to this dilemma, Wear Technology Inc., has designed an erosion test apparatus and method to rapidly perform erosion tests. A prototype of this design is presently being built for test. The rate of erosion test output will be approximately 5 to 10 times higher with the new method than with existing jet impingement testers. This higher test output will make generation of erosion wear data bases feasible. Wear Technology intends to produce and publish erosion data banks on engineering materials, including metal alloys, ceramics, cermets, composites, coatings, polymers, surface modifications, and hardface materials.

Erosion-Slurry

One of the most meaningful types of tests for evaluating the abrasive wear resistance of coatings is slurry erosion tests. Many applications of coatings are found in equipment and components handling slurry systems; i.e., pumps, valves, piping, hydrocyclones etc. Many types of slurry erosion testers are used including liquid jet impingement, slurry pot with stirrer, and slurry pot with impeller. The last will be be described in this report.

Bureau of Mines Slurry Tester

A unique slurry tester shown in Figure 14 was developed by the Bureau of Mines for studies of materials for slurry-handling equipment in the mineral industry (6). Several features that make this apparatus versatile are; choice of recycled slurry operations or "once-through" slurry flow, eight-specimens tested simultaneously, specimens are simple rectangular blocks that can be easily coated, particle velocity can be varied, and a wide variety of slurry liquids and abrasives can be selected. Also electrodes attached to specimens allow potentio-static corrosion studies to be made along with erosion studies. Thus, erosion and corrosion effects can be measured separately or combined,, as desired. Researchers at the Bureau of Mines are presently studying some very unusual synergistic effects of erosion/corrosion with the slurry tester.

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