Im Hutchings

Transitions between two-body and three-body abrasive wear: influence of test conditions in the microscale abrasive wear test

Abstract The microscale abrasive wear test (also known as the ball-cratering wear test) is generally considered to be a three-body wear test. Nevertheless, different test conditions can produce either two-body (grooving) or three-body (rolling) wear mechanisms. The wear mechanisms and wear rates were investigated over a range of loads (0.1 to 5.0 N), slurry concentrations (0.000031 to 0.24 volume fraction abrasive) and abrasive materials (SiC, Al 2 O 3 and diamond). It was found that for each abrasive, a transition from grooving to rolling wear could be identified with a critical ratio of load to slurry concentration. The wear rate varied with concentration, with a maximum at intermediate slurry concentrations. The classification of abrasive wear into two-body and three-body mechanisms is discussed with reference to the problems noted by Gates [J.D. Gates, Two-body and three-body abrasion: a critical discussion, Wear 215 (1998) 139–146].

A micro-abrasive wear test, with particular application to coated systems

A test methodology is described which permits a rotating sphere test, also used to measure coating thickness, to be used as a small-scale abrasive wear test. The ability of the test to measure intrinsic wear resistance for thin coatings is demonstrated together with a new method of analysis which allows simultaneous evaluation of the wear resistance of both substrate and coating from their combined wear behaviour in a single test. The method has been applied to physically vapour deposited TiN, TiCN and TiAlN coatings on tool steel substrates, as well as to bulk samples of metals, ceramics and soda-lime glass.

Theory and application of a micro-scale abrasive wear test

A micro-scale abrasion test has recently been developed that allows measurement of the wear resistance of the surface regions of a material. The typical penetration depth is less than 30 μm. The test uses a simple mechanical and optical system and involves rotating a hard steel sphere against a specimen in the presence of small abrasive particles. The method has been used to investigate the wear resistance of thin PVD coatings (1 to 5 µm), metallic glass ribbons, and paint films in addition to bulk samples of metals, ceramics, and glasses. The associated theory has been extended so that results may be obtained from any curved surface. This furthers its applicability to practical surface-engineered components such as twist drills, bearings, turbine blades, and biomedical prostheses. A detailed characterization of the experimental procedure has been undertaken to provide an understanding of the repeatability and sensitivity of the test. In a study of cutting-tool coatings, the wear resistance measured by this method has been shown to correlate with scratch test response and with the performance of coated end-mills in cutting tests.

Wear-mode mapping for the micro-scale abrasion test

Abstract The objective of this study was to develop a theoretical model and associated wear-mode map to identify the regimes in which two-body abrasion (grooving abrasion) and three-body abrasion (rolling abrasion) dominate in the micro-scale abrasive wear test (also known as the ball-cratering abrasion test). The critical condition for the transition between two-body and three-body abrasion was determined from a continuum mechanics model for the penetration of the abrasive particles into the surfaces of the ball and the specimen, coupled with considerations of equilibrium. Micro-scale abrasion tests were performed with different combinations of ball and specimen materials, under different test conditions such as abrasive concentration and load, and a wear-mode map has been produced which defines the regimes of abrasive particle motion. The map is plotted between two dimensionless groups as vertical and horizontal axes: the hardness ratio between the ball and the specimen, and a newly introduced parameter which represents the severity of contact. Experimental data generated in this work and also taken from previous studies show that the map represents behaviour in the micro-scale abrasion test well, for a wide range of ball and specimen counterface materials.

Wear resistance of amorphous alloys and related materials

Abstract Amorphous alloys, and the partially or fully crystallised materials derived from them, can have properties attractive for a diverse range of applications. In some cases, their wear resistance can be of primary concern, in others, it is an important secondary property. The distinctive mechanical properties of amorphous alloys make their wear resistance of fundamental interest also. This review focuses on the influence of a variety of factors in wear testing, on the mechanisms of wear, on the characteristics of different categories of amorphous alloy, and on the effects of partial or complete crystallisation. It is shown that amorphous alloys can have very good resistance to sliding and abrasive wear. The wear resistance of related quasicrystalline phases is also considered.

Progress towards standardisation of ball cratering

The ball cratering (micro-abrasion) test is becoming popular as a method for the abrasion testing of surface engineered materials. It possesses many advantages over more conventional abrasion tests including the ability to test small volumes of material and thin coatings, its perceived ease of use and the low cost of the test equipment, and its versatility. Standards are now being drafted both in the USA and Europe on ball cratering, but further work is needed before this work can be completed on the effect of test variables and the choice of measurement method on the results that are achieved. This paper discusses these aspects of the test and its relevance to industrial wear problems, and describes the results of a preliminary interlaboratory exercise that has been conducted in the UK to determine the effectiveness of the test method. The paper will also give an outline of an EU funded project that has the aim of validating the test and which brings together a consortium of 10 research partners from four European countries.

Design of a slurry erosion test rig

Abstract The design of a jet impingement slurry erosion test rig, built for laboratory use, is presented. This apparatus gives good control over many of the important test parameters, such as impact velocity, solid particle concentration and impact angle. An ejector nozzle is employed to entrain sand particles from a sand bed into a stream of water to form a slurry; after impingement, the abrasive particles and the water phase are separated and recycled. This makes the rig simple, economical and easy to operate and its pump and pipeline remain free from erosive wear. Experimental results are presented to illustrate the operation and performance of the rig.

Solid particle erosion studies using single angular particles

Abstract Studies have been made of the oblique impact of individual angular particles on lead and mild steel targets, and the influence of the particle orientation at the point of impact on the subsequent deformation determined. When the angle made by the leading edge of the particle to the surface is small, ploughing deformation can occur, similar to that found with spherical particles. At a sufficiently high impact velocity, material is removed from the surface. If the angle between the surface and the leading edge of the particle is larger, a micromachining action occurs. However, rather than scooping out material as a chip, the cutting edge of the particle tends to bury itself deeply into the specimen. Material can be removed as a result of a particle breaking up during its cutting action. Here, a lip raised during the early stages of the impact is subsequently cut off by fragments of the particle. It is shown that particle rotation can take place during impact and that when this happens the particles effectiveness in removing material is diminished. Bands of intense deformation are visible in etched sections of craters in mild steel formed in some experiments. It is suggested that these are adiabatic shear bands.

The rôle of particle properties in the erosion of brittle materials

The erosion of a range of brittle materials (soda-lime glass, borosilicate glass, fused silica, boron carbide, partially stabilized zirconia, alumina and silicon carbide) with number of different erodent particle types (silicon, silica, alumina and silicon carbide) has been examined. The mechanisms of erosion depend upon the ratio of particle to target hardness. As this ratio falls towards unity, less damaging mechanisms of erosion dominate. Indentation-induced fracture models for erosion of brittle materials are not valid when this mechanism ceases to operate. The erosion rates decrease rapidly, and the velocity exponents of erosion rate increase, as the ratio of particle to target hardness decreases towards unity. The consequences of this in accelerated erosion testing are addressed.

Inkjet printing - the physics of manipulating liquid jets and drops

Over the last 30 years inkjet printing technology has been developed for many applications including: product date codes, mailing shots, desktop printing, large-area graphics and, most recently, the direct writing of materials to form electronic, biological, polymeric and metallic devices. The new non-graphical applications require higher print rates, better resolution and higher reliability while printing more complex, non-Newtonian and heavily solids-loaded liquids. This makes the understanding of the physics involved in the precise manipulation of liquid jets and drops ever more important. The proper understanding and control of jet formation and subsequent motion of the jetted materials requires physical studies into material properties at very high shear rates, acoustic modes in print heads, instabilities of jets, drop formation, drop motion, stretching of fluid ligaments, the role of polymers in jet break up, electrical charging of drops and the aerodynamic and electrostatic interaction of jets and drops in flight. Techniques for observation, measurement and analysis are evolving to assist these studies. This paper presents some examples of the application of physics to understanding and implementing inkjet printing, including recent work at the Cambridge Inkjet Research Centre.