N.W. Page

Wear mechanisms in polymer matrix composites abraded by bulk solids

Abstract An experimental study of the wear of polymer matrix composite materials subjected to abrasion from bulk materials has been conducted. Three examples of vinyl ester resin systems were considered: (a) unreinforced, (b) reinforced with glass fibres, and (c) reinforced with particles of ultra high molecular weight polyethylene (UHWMPE). Soft and hard bulk materials used for abrasion were granular forms of coal and the mineral ignimbrite. The bulk material was presented to the wear surface on a conveyor belt in a novel wear tester. While UHWMPE reinforcement enhanced the wear resistance to both hard and soft abrasives, the situation for fibre reinforcement was more complicated. With coal as the abrasive, it was found that glass fibre reinforcement reduced the wear rate, whereas in the case of the harder ignimbrite, fibre reinforcement increased the wear rate. Microscopy indicated significant differences in the mechanism of wear in each surface/abrasive combination. Wear textures, consistent with both two and three-body wear, were observed with, respectively, soft and hard abrasive particles.

Effects of graphite particle addition upon the abrasive wear of polymer surfaces

The abrasive wear performance of vinyl ester resins modified with various volume fractions (5, 10, 15, 20 and 30%) of graphite powder has been measured. Using a conveyor belt driven testing machine developed locally, it has been possible to realistically simulate the effect of three-body abrasive wear upon these graphite modified polymer samples. A comparison of the calculated dimensionless wear rates obtained for these surfaces reveals that the effect of the graphite powder depends strongly upon the volume fraction of particles in the resin matrix. It appears that, for intermediate volume fractions, the presence of graphite powder in the resin matrix reduces the abrasive wear of the polymer surface. Scanning electron microscopy has been used to probe the mechanisms of abrasive wear of the pure resin and graphite modified surfaces. It appears that the embedded graphite particles can act as a lubricant during the abrasion process thus reducing the wear rate. The effect of increasing graphite powder volume fraction upon the abrasive wear mechanism is discussed.

Elastic properties of compacted metal powders

This paper describes a study of the unloading characteristics of compacts made from the uniaxial compression of metal powders in a cylindrical die. Spherical, irregular and dendritic copper powders and spherical stainless-steel powder were investigated to determine size, shape and material effects on the unloading response. This response was characterized in terms of Youngs modulus and Poissons ratio. Measures of these quantities were made at different relative densities by unloading from different peak axial stresses. With both parameters, there was a strong dependence on particle shape. The load response of lightly compressed material was found to be dominated by its particulate nature and interparticle forces. Unloading material in this condition gave values of Youngs modulus that increased slightly and Poissons ratio that decreased with increasing values of relative density. In contrast, the load response of heavily compressed material was found to be similar to that of a porous solid. Unloading material in this condition gave values of Youngs modulusthat increased more steeply and Poissons ratio that increased with increasing values for the starting relative density. Transition between these two types of behaviour depended on the particle shape, and also, to a lesser extent, the particle material.

A polyethylene-reinforced polymer composite abraded by bulk solids

The abrasive wear characteristics of polymer matrix composite materials reinforced with particles of ultra-high molecular weight polyethylene (UHMWPE) have been investigated. Granular ignimbrite (a hard mineral) was used as the abrasive. The effect of varying the volume fraction of UHMWPE reinforcement upon wear rate has been measured and scanning electron microscopy (SEM) used to study the wear mechanisms that operate. The results demonstrate that there is a distinct transition that occurs in the dominant wear mechanism as the volume fraction of UHMWPE reinforcement is increased. In particular, as the UHMWPE concentration in the resin is increased, friction at the surface decreases and the wear mechanism appears to change from predominantly three- to two-body wear, with a corresponding decrease in wear rate. This transition in mechanism appears to occur when the wall friction drops below the internal friction of the granular bed. At higher values of wall friction, the plane of shear failure is within the granular material so that the particles in contact with the surface have a rolling component across the surface. At lower values of wall friction, the plane of shear failure occurs at the surface.

Modelling the three-body abrasive wear of UHMWPE particle reinforced composites

Abstract In this paper, some of the complexities in developing predictive models for polymer composite materials are discussed with particular reference to models based on surface deformation energy. Although there are models available for abrasive wear of metals, polymers and composite materials, models are suitable for a particular wear situation. Most of the cases, the wear models are based on traditional test methods, such as two-body abrasive wear test or tests in a controlled environment. Moreover models for polymer composite materials are very limited. This is due to the fact that the vast range of polymers and polymer composite materials present different wear behaviour in similar wear situations. This makes the modelling wear of polymer composite materials complicated. This paper presents and discusses recent test results on ultra high molecular weight polyethylene (UHMWPE) particle reinforced composites with varying particle concentration from a unique test rig that exposes the wear surface to sliding bulk solids (granular materials). The wear phenomena of resin and UHMWPE particle reinforced resin surfaces abraded by bulk solids are discussed with respect to the interaction between the particles and the wear surfaces. The effects of elastic and plastic deformation energy of the surfaces on the abrasive wear resistance are also discussed in the context of an appropriate wear model for these surfaces and wear events. Finally a model is proposed to predict the wear of these polymer composites based on the deformation energy of the surfaces. Good qualitative agreement was obtained between the model and the experimental wear rates.

A high-pressure shear cell for friction and abrasion measurements

This paper describes the design and operations of a high-pressure shear cell capable of pressing granular material against candidate wear surfaces at macroscopic pressures of up to 700MPa. The wear surface is then forced laterally yielding information about coefficient of friction and wear mechanics during slip at the surface. This pressure covers the range commonly experienced in many mining operations, including those involved in ore crushing. Recent results with the shear cell will be presented. This involves the crushing of quartz against test tiles of Ni-hard 4 steel. Results for coefficient of friction and 3-body abrasion are reported. Wear results are quantified and compared in terms of contact profilometry and non-contact profilometry involving reflected light confocal microscopy.

Friction measurement on Ni-Hard 4 during high pressure crushing of silica

Abstract A novel high pressure shear cell has been used to study friction phenomena between crushed silica and Ni-Hard 4 at surface stresses commonly experienced in ore crushers. Surface normal loads of more than 300xa0MPa were used, together with tangential loads sufficient to initiate and maintain slip at the crushing surface. Under these load conditions, the friction coefficient increased with sliding distance, rising from an initial value of less than 0.1 and leveling off to a plateau value of about 0.4–0.6 in the first few millimeters of shear displacement. The variation of friction coefficient with shear displacement could be divided into three stages: the first dominated by particle rearrangement in the abrasive bed, a second transition stage where particle rearrangement and particle crushing occurred together, and a third dominated by a slowly evolving layer of fine powder adjacent to the crushing surface. This last stage was characterized by a relatively stable (plateau) value for the friction coefficient. This plateau was a weak function of the normal load. Analytical and experimental results showed that larger abrasive particles preferentially penetrate the surface and, as a result, cause the most extensive ploughing damage. Thus, if fine particles are concentrated at the crushing surface they can protect that surface from penetration and ploughing by larger particles, a feature also observed experimentally. An energy analysis of the crushing and wear event revealed that interparticle friction and particle fracture occurring within the bed of abrasive material contributes significantly to the energy consumed as a result of shear displacement.

Influence of comminution products on abrasive wear during high pressure crushing

A novel high pressure shear cell (HPSC) has been used to study comminution and wear behaviour at surface stresses commonly experienced in ore crushers. In this work, quartz and Ni-hard 4 were chosen as representative ore and wear surface materials. Surface normal loads of more than 300 MPa were used, together with tangential loads sufficient to initiate and maintain slip at the wear surface. Comminution was studied in terms of overall particle size distribution and size distribution in the crushing zone. Worn surfaces were studied using optical, confocal and scanning electron microscopy, and contact profilometry. There is a strong coupling between comminution outcomes and wear. When crushed ore is subjected to a shear stress at the wear surface, this shear stress increases the principal stress so that a layer of fine powder is produced against the wear surface. By appropriate control of the normal/shear stress cycle, this can lead to a protective layer of fine material against the wear surface which inhibits larger particles from becoming embedded in the surface, thus reducing both ploughing and cutting wear mechanisms. This can lead to reduced wear by ploughing and cutting at higher surface normal loads compared to the case of lower normal surface loads.

Hot shock compaction of nanocrystalline alumina

An experimental investigation of hot shock compaction of a nanocrystalline alumina powder was performed. The effects of variations in shock pressure and compaction temperature on the properties of the compacted materials were studied. It was found that the bulk density and hardness of the compacted material increased with shock pressure. Increasing compaction temperature resulted in increases in compact hardness and bonding, and reductions in cracking within the compacted specimens. The results suggest that dense, well bonded, crack free nanocrystalline ceramics may be fabricated more effectively using hot shock compaction, than by room temperature shock compaction followed by sintering or room temperature static compaction followed by sintering.

Flow Visualisation in Dense Phase Pneumatic Conveying of Alumina

Pipeline wear is a very complex problem and at present there is limited understanding of the wear mechanisms responsible for the reduction of pipe wall thickness in critical wear areas. The ability to determine the wear mechanisms in these areas holds the key in determining the service life of pneumatic conveying pipelines in industry. In this paper design and construction of a visualisation test rig is presented which enables better understanding of the flow process in pneumatic conveying pipelines. Results from the flow visualisation have been analysed and presented for an insight to the flow patterns in the critical wear areas of pneumatic conveying pipelines.