M.G. Gee

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.

The formation of aluminium hydroxide in the sliding wear of alumina

Abstract The wear of ceramics is often affected by tribochemical reactions with the surrounding environment. This paper examines the development of aluminium hydroxide interfacial layers in the wear of alumina at intermediate wear rates and the effect of these layers on wear and friction. Since the humidity of the surrounding atmosphere promotes the formation of these layers, a dramatic effect of humidity on the wear rate is brought about by the changes in layer formation. In supporting experiments, it was found that alpha-alumina reacts directly to form boehmite (Al(OH)) under moderate pressures and high temperatures.

The combined effect of speed and humidity on the wear and friction of silicon nitride

Abstract A series of ball-on-ring wear tests was performed on sintered silicon nitride. The experiments were carried out to investigate a discrepancy in the literature on the effect of humidity on the wear of silicon nitride. In the experiments reported here, a range of different testing speeds was used and the humidity of the tests held at less than 5% relative humidity (RH), 50% (RH), or 90% (RH). It was found that at the lowest speed that was used, the wear rate at low humidity was higher than that for higher humidity. As the speed increased there was a maximum in the wear rate for the high humidity tests, and a minimum for the low humidity tests, with wear rates for the low humidity tests much lower than for the high humidity tests. At high speeds, the wear rate for the low humidity tests was much lower than that for the high humidity tests. The main mechanism of wear was the tribochemical oxidation of the silicon nitride to form silicon oxide.

Low load multiple scratch tests of ceramics and hard metals

Abstract Abrasion is caused by the repeated scratching of materials by individual particles in an abrasive, often under fairly light loads. This process has been simulated by carrying out scratch tests on a range of ceramics and hard metals. An array of different scratches was carried out on each sample with a different number of repeats along the same track for each scratch. The magnitude of the damage was measured by the width of the scratches. The frictional force between the indenter and test sample was also measured. Although the width of single pass scratches in some of the harder materials was smaller than in softer materials, in multiple pass scratches, the final widths of scratches in some of the harder materials were greater than in the softer materials. This was due to differences in the contribution of fracture in the development of damage in multiple pass scratches. It was found that fracture was a predominant form of damage to both hard metals and ceramics. In the case of the hard metals the fracture was on a fine scale, but with the ceramics fracture occurred on a larger scale, often removing large fragments of material. These results, and the results of the friction measurements are correlated with the results of a microstructural examination of the mechanisms that occurred. They are also compared with a microstructural assessment of the early stages of wear in the abrasion of these materials.

The European nanometrology landscape

This review paper summarizes the European nanometrology landscape from a technical perspective. Dimensional and chemical nanometrology are discussed first as they underpin many of the developments in other areas of nanometrology. Applications for the measurement of thin film parameters are followed by two of the most widely relevant families of functional properties: measurement of mechanical and electrical properties at the nanoscale. Nanostructured materials and surfaces, which are seen as key materials areas having specific metrology challenges, are covered next. The final section describes biological nanometrology, which is perhaps the most interdisciplinary applications area, and presents unique challenges. Within each area, a review is provided of current status, the capabilities and limitations of current techniques and instruments, and future directions being driven by emerging industrial measurement requirements. Issues of traceability, standardization, national and international programmes, regulation and skills development will be discussed in a future paper.

High resolution characterisation of tribochemical films on alumina

Abstract This paper presents the results of the characterisation of tribochemical films produced by the sliding wear of alumina in a humid environment. The techniques that were used were field emission scanning electron microscopy (FESEM) mechanical properties microprobe or nanoindentor (MPM), atomic force microscopy (AFM), frictional force microscopy (FFM) and non-contact optical profilometry. It was found that the tribochemical films were softer than the remnant alumina on the worn surfaces, and were liable to fracture and cracking under the indenter. The FESEM showed similar features to the AFM, but because of its direct height imaging, the AFM was better at resolving and imaging small MPM indentations. The AFM also showed that the tribochemical films were composed of small rounded particles which were pressed together to form the films. Some FFM contrast is attributed to steep gradients or edges, and some to variations in surface roughness. There is, however, some contrast which cannot easily be explained by simple geometric interactions and appears to be associated with variations in surface chemistry and structure.

Wear of tungsten carbide–cobalt hardmetals and hot isostatically pressed high speed steels under dry abrasive conditions

Abstract Tungsten carbide–cobalt (WC–Co) hardmetals and tool steels are materials that are widely used in applications where abrasion resistance is required. This paper describes the results of tests that were performed using a modified ASTM dry sand rubber wheel test system. It was found that although there was only a small increase in wear as the relative speed increased, a sudden increase in wear of two orders of magnitude was observed as the load was increased for both the WC–Co hardmetals and the tool steels. It was also interesting that under most test conditions the tool steel materials wore at the same rate as the WC–Co hardmetal of equivalent hardness. For all the materials the most noticeable difference was that under high applied load there was significant macroscopic grooving of the worn surfaces. By contrast the worn surfaces were dull but smooth under all other test conditions. When examined at high magnification, there was little difference in the appearance of the wear surface from one test condition to another. In the WC–Co hardmetals, the binder phase had been removed from the surface layers, and there was evidence for intragranular fracture to the WC grains. In the tool steel, decohesion fracture frequently occurred at the trailing edge of carbide particles. These results are complemented by examination of subsurface damage through cross-sectioning.

High temperature friction and wear testing of silicon nitride ceramics

This paper describes the results of unlubricated sliding wear tests carried out on ceramics and composites based on silicon nitride. These were silicon nitride, silicon nitride with titanium carbide, and an electrically conductive silicon nitride with titanium nitride. The tests were carried out from room temperature to 1200°C. For all the materials, there was a small increase in friction from room temperature to 300°C, followed by a small decrease as the temperature increased, until a sharp increase in friction coefficient at 1200°C. There was a maximum in wear as the temperature was varied for all materials, with almost no wear at room temperature and at 1200°C. The wear of the silicon nitride material was significantly higher than that of the composite materials at 900°C.

Processing of an ultrafine-grained titanium by high-pressure torsion: an evaluation of the wear properties with and without a TiN coating.

A commercial purity (CP) Grade 2 Ti was processed by high-pressure torsion (HPT) using an imposed pressure of 3.0GPa at room temperature. The HPT processing reduced the grain size from ∼8.6 μm in the as-received state to ultra-fine grains (UFG) of ∼130 nm after HPT. Tensile testing showed the HPT-processed Ti exhibited a good combination of high ultimate tensile strength (∼940 MPa) and a reasonable elongation to failure (∼23%). Physical vapour deposition was used to deposit TiN coatings, with a thickness of 2.5 μm, on Ti samples both with and without HPT processing. Scratch tests showed the TiN coating on UFG Ti had a critical failure load of ∼22.5 N whereas the load was only ∼12.7 N for the coarse-grained Ti. The difference is explained using a simple composite hardness model. Wear tests demonstrated an improved wear resistance of TiN coating when using UFG Ti as the substrate. The results suggest that CP Ti processed by HPT and subsequently coated with TiN provides a potentially important material for use in bio-implants.

Effects of surrounding atmosphere on the wear of sintered alumina

Abstract The dry-sliding wear of polycrystalline sintered alumina was found to increase by nearly one order of magnitude when the relative humidity (RH) of the surrounding air decreased from 50% to 10% RH and by a further order of magnitude as the humidity decreased still further down to 4% RH. Increasing the humidity above 50% RH had little effect on the amount of wear. While intergranular fracture is present at all humidity levels, below 50% RH transgranular fracture became increasingly common. At humidities of 10% RH and above wear was reduced by the formation of a film of material with reduced aluminium content, possibly hydrated alumina. Below 10% RH the film was not formed, resulting in the large wear rate observed at these low humidity levels. Another possible reason for the reduction in wear is the facilitation of plastic flow by the increase in humidity. This may have accommodated the contact shear stresses so that they were reduced below the level at which transgranular fracture was initiated. These effects have important consequences for the wear testing of alumina and for the design of components for antiwear applications.