Test method for wear resistance of surface coatings

Wear occurs to the hardest of materials, including diamond, wear studies having focused on surface damage in terms of material-removal mechanisms, including transfer film, plastic deformation, brittle fracture and tribochemistry.
With the development of surface engineering design, the need to evaluate the properties of new raw materials and substrate-coating combinations is important. In many research works to date, the authors have investigated the effects of contact abrasion, erosion and impact effects on uncoated components, mainly as separate problems [2]. More recently,experiments and testing on coated materials have occurred and some standardised, and experimental test equipment has been produced to meet specifications on wear resistance. Standard test methods such as pin-ondisc are used extensively to simulate rubbing action in which plastic yielding occurs at the tip of individual asperities. This testing is mainly carried out on a microscopic scale and in thin films technology.

Wear test methods

Tests are used for quality control functions such as thickness, porosity, adhesion, strength, hardness, ductility, chemical composition, stress and wear resistance.
Non-destructive tests include visual, penetrant dies,magnetic particle and acoustic techniques. Many tests for coated and uncoated cutting tools are conducted on machine tools, including lathes, mills, drills, punches and saws [13,14]. These test methods provide almost identical conditions to those experienced in manufacturing. Machining tests subject cutting tools to many wear parameters, including impact and shock, abrasion,adhesion and hot corrosion. The limitations of these tests depend on the machine power available and the quality of the machine tool. Other coated components that are not used as cutting tools are assessed by laboratory wear tests and compared to field studies.
Such equipment includes nano- and micro-hardness testers, fatigue testers, acoustic, and scratch-type test equipment, etc.

Abrasi6e and adhesi6e test equipment
Hardness is often used as an initial guide to the suitability of coating materials for applications requiring a high degree of wear resistance. The effect of the hardness of a wearing material however is complicated,as different wear mechanisms can prevail in service.
Scratch hardness is the oldest form of hardness measurement. Mohs in 1822 categorised materials using this process, giving diamond a maximum scratch hardness of ten. Most scratch type tests developed from this simple technique. Abrasive tests are described by Kato et al. [15] and others [16,17]. Adhesion is characterised by both scratch- and indentation-tests as reported in the literature [18,19]. In indentation adhesion tests, a mechanically stable crack is introduced into the interface of the coating and substrate. The resistance to propagation of the crack along the interface is used as a measure of adhesion. In scratch-adhesion tests, a stylus is drawn over the surface under a continually increasing normal load until the coating fails. Factors influencing the wear mechanisms during sliding contact are shown in Fig. 3 and the metallurgical properties influencing sliding wear are shown in Fig. 4.

Pin-on-disc
Research conducted by Glaeser and Ruff reported that pin-on-disc were the most widely used wear test processes, followed by pin-on-flat [20]. Other applications of pin-on-disc include material wear and friction properties at elevated temperatures and in controlled atmospheres [6]. Almond et al. [21] used a pin-on-disc apparatus for testing ceramics and cemented carbides on alumina discs using the pin as the test material. In a two-body abrasion test, a coated pin is pressed against a rotating abrasive paper making a spiral path to avoid overlapping [22,23]. This test process is very common for thin coatings. Using a diamond tip as the abrading tool, Kato et al. [15] used a pin-on-disc test to operate within the chamber of a Scanning Electron Microscope(SEM) to examine abrasion effects. Scratch testing in conjunction with SEM provides a useful method of analysing single-point wear mechanisms of coated systems through an assessment of the deformation and fracture produced.

Pin-on-drum abrasi6e wear test
In this test, one end of a cylindrical pin specimen is moved over abrasive paper with sufficient load to abrade material from the specimen and crush the fixed abrasive grains. This test simulates the wear that occurs during crushing and grinding of ore in which the abrasive (the ore) is crushed. The pin also rotates while traversing, as indicated in Fig. 5. This ensures that the pin always contacts fresh abrasive. This is a high-stress abrasion test, as the load is sufficient to fracture the abrasive particles.

Repeated impact wear test
Equipment described by Blickensderfer and Tylczak[24] involved balls made from alloys being dropped 3.4m onto a column of balls, with each successive ball receiving an impact on each side. The first ball receives maximum impact whilst the last one receives the least.
This rig, as shown in Fig. 6, tests materials for spalling due to impact and shock only. It does not take account the orientation of the samples, which latter can be up to 50 mm diameter. The samples are also subjected to rebound, which gives a double-impact effect. An impact testing machine for determining the dynamic cushioning properties of plastic foams is reported by Shestopal and Chilcott [25] and shown in Fig. 7. This process is pure impact and has many limitations, as described in the reference. Brenner et al. [26] used a test rig to combine impact and its effect on adhesion at elevated temperatures for iron spheres impacting on an iron plate. The impact forces are transmitted to piezoelectric load cells, which produces a pulse on a screen that equivalent to the applied load.

Adhesion tests using acoustic amission monitoring Experiments conducted by Diniz et al. [27] used acoustic emission to monitor the changing of workpiece surface roughness caused by an increase in tool wear during finish turning. Adhesion tests conducted by Kubo and Hashimoto [28] made use of a modified scratch test with a steadyily-increasing load. This test is designed to evaluate thin film properties such as adhe-sion [29,30], and the critical load at which the film becomes detached is detected by acoustic emission. A diamond indenter tip is normally employed in this test,along with a camera and SEM to observe how the films are scratched. Fig. 8 presents a schematic representation of a scratch coating adhesion test using acoustic emission.

Rubbing tests
An ASTM standard [31] uses a crossed-cylinder apparatus for testing similar and dissimilar metals, and alloys and coated systems under unlubricated conditions. A rotating cylinder (100 rpm) is forced at right angles against a stationary cylinder. The volume of material loss is determined by means of an appropriate equation.

Block-on-ring test
This test, ASTM G77-83 [32], makes use of a rotating metal ring acting against a fixed block. It makes a line contact when the test begins, as shown in Fig. 9. This test allows variations in materials, speeds, loads, lubricants, coatings and different operating atmospheres.
Wear is calculated using the volume loss of the block and the weight loss of the ring.

Taber test
The Taber Abraser, ASTM 1044, is used to measure the low-stress abrasive wear resistance of materials and coatings. Low-stress abrasive wear occurs when hard particles are forced against and move along a flat, solid surface where the particle loading is insufficient to cause fracture of the hard particles. Two- and threebody abrasive wear can be assessed with this method.
The Taber apparatus is shown in Fig. 10. The specimen, which is coated or uncoated, is rotated, causing the abrasive wheels to drag and abrade the surface. Wear is normally determined by weight loss.

Dry sand rubber wheel test
This test, ASTM 65-81 is used to rank the abrasion or scratch resistance of materials to silica sand. It is a low-stress abrasion test and is used for dry wear conditions. In operation, sand particles are trapped between the specimen and a rubber wheel and dragged along as the wheel rotates. The specimen is held against the wheel with a contact force. Cerri et al. [33], using similar equipment, examined the abrasion resistance of carbide powders with several materials and coatings used for applications in abrasive environments. Swanson [34] used a dry-sand rubber wheel test to compare laboratory and field tests under sandy-soil working conditions, and concluded that there was a close correlation between the two.

Alumina slurry test
An alumina slurry test, standard ASTM 611, is used to simulate high abrasive conditions in a liquid medium[35]. The test rig is shown in Fig. 11. It uses a steel wheel which rotates against a flat coated specimen in a slurry containing sharp alumina particles, subjecting the samples to combined impact erosion. Impact by particles causing erosive wear are described in Fig. 12. The effects of erosion have become a problem associated with airfoils and shrouds in various fans, in compressors and turbines, on helicopter blades, in centrifugal pumps, on valve components and in pipe joints and bends [6]. The extent of erosion depends on the composition, size, and shape of the eroding particles, their velocity and angle of impact, and the composition and microstructure of the surface being eroded.

Prev: Laboratory wear testing method