Niklas Axen

A model for the abrasive wear resistance of multiphase materials

Abstract A theoretical model for estimating the abrasive wear resistance of multiphase materials and composites from the wear resistance of the constituents is proposed. Starting from Archards equation, two rules for the dependence of the amount of reinforcing phase, corresponding to two wear modes, have been derived. These rules of mixture, constitute estimations of the upper and lower limits for the wear resistance of a composite. The theory also results in two corresponding formulations of the wear equation generalised to multiphase materials. The applicability and limitations of the theory are discussed. The validity of the theory is supported by results from two test series. Three-body abrasive tests with silicon carbide abrasives were performed on simple specially fabricated model composites and silicon carbide particle-reinforced aluminium alloy composites were tested in two-body abrasion against silicon carbide and flint abrasives. The model serves to clarify the roles of the matrix and the reinforcing phase of a composite in resisting wear. The model can be instrumental in estimating upper and lower limits of wear and in indicating how to redesign a composite or modify a tribosystem (changing load, filtering abrasive particles, etc.) to transform the wear of a multiphase material towards the least severe mode.

An evaluation of plasma sprayed ceramic coatings by erosion, abrasion and bend testing

Abstract The mechanical properties of a series of plasma sprayed ceramic coatings made from alumina, chromia and alumina–titania precursor powders have been evaluated in particle erosion, two-body abrasion and bend testing. For the alumina coatings, both monocrystalline (sapphire) and polycrystalline alumina powders of varied grit sizes were used. Also, coatings of chromia and a mixture of alumina and titania were evaluated. The coatings were deposited with an axial injection spraying technique. Clear differences were found between the investigated coatings. Among the alumina coatings, those deposited from sapphire powders gave the best results in the wear tests. An addition of titania to alumina had a favourable effect in most tests. In abrasion, one chromia coating displayed a wear resistance comparable to the dense sintered alumina reference. The erosion rates were, however, several times higher for all sprayed coatings compared to the reference alumina.

Surface damage on cemented carbide rock drill buttons

Abstract Wear phenomena on cemented carbide drill buttons tested in percussive drilling of Kiruna-magnetite under realistic conditions have been studied with scanning electron microscopy, light optical microscopy and energy dispersive X-ray spectroscopy. In addition, a comprehensive literature study has been performed. The aim has been to chart wear phenomena, such as crack formation, occurrence of reptile skin and various wear mechanisms. It is shown that surface cracks form shortly after drilling has started. Also the valleys and plateaus characterising the reptile skin pattern were found after only short drilling periods. It appears that the formation of the initial surface cracks and the reptile skin pattern are separate mechanisms, which presumably interact. In addition to the formation of cracks, there is also a continuous wear of the buttons. This is assumed to be caused by wear fracturing and pullout of tungsten carbide (WC) grains and wear of cobalt (Co) by abrasion, possibly assisted by corrosion. A wear mechanism table, based on a comprehensive literature review and on experience from real drilling conditions, is suggested.

Cohesion in plasma-sprayed coatings - A comparison between evaluation methods

Abstract The integrity, and especially, the cohesive strength of alumina plasma-sprayed coatings prepared from a range of precursor powders of different size and crystallinity has been investigated by several different methods. Several chromia and alumina-titania coatings, as well as a sintered bulk alumina, have also been studied for comparison and reference purposes. Methods of characterisation have included assessment of the coatings resistance to open tribo-system wear by dry particle erosion and abrasion on the one hand, and their resistance to mechanical failure during controlled scratch testing and tensile fracture in four-point bending, on the other. Controlled scratching was carried out on both top- and cross-section, with the cross-sectional scratching done inside a scanning electron microscopy (SEM). Bending was carried out, in ambient conditions, using test equipment small enough to be placed in the SEM, in order to observe the progressive cracking behavior and failure of the coatings. Some correspondence was found between most of the techniques/methods studied, but erosion and abrasion with large, hard particles and scratch testing on both top surfaces and cross-sections provided the best correlation. It is concluded that these are the most promising methods found so far to rank the cohesion characteristics of lasma sprayed coatings.

Tribofilm formation on cemented carbides in dry sliding conformal contact

Abstract The friction properties and the tribofilm formation of a binderless cemented carbide and two conventional cemented carbides have been evaluated in an unlubricated sliding contact in either air or nitrogen surroundings. A continuously varied normal load and two rotational speeds were used. The tribofilms were analysed with SEM, XPS, TEM and EELS. For all cemented carbides, friction was lower at higher sliding speed. The two conventional carbides also tended to give a lower friction in air than in nitrogen, whereas the binderless carbide gave about the same friction in both environments. All materials formed tribofilms during the tests. Mild tests with the conventional cemented carbides resulted in relatively thick tribofilms (a couple of μm) with low amounts of oxides. At more severe test conditions the tribofilms became thinner (

Abrasive wear of alumina fibre-reinforced aluminium

Abstract The friction and abrasive wear behaviour of an Al-Si1MgMn aluminium alloy reinforced with 10, 15 and 30 vol.% of alumina fibres have been evaluated. The influence of fibre content, matrix hardness, applied load as well as the hardness and size of the abrasive grits was investigated. The tests were performed with a pin-on-drum two-body abrasion apparatus. The wear mechanisms were studied using scanning electron microscopy. It is shown that fibre reinforcement increases the wear resistance in milder abrasive situations, i.e. small and soft abrasives and low loads. However, in tougher abrasive situations, meaning coarse and hard abrasives and high loads, the wear resistance of the composites is equal to or, in some cases, even lower than that of the unreinforced material. It is also shown that the coefficient of friction decreases with increasing fibre content and matrix hardness of the composites.

A model for the hardness of cemented carbides

Abstract A new model for the hardness of cemented carbides is proposed. The model is based on the main assumption that very thin binder layers (a few atom layers) confined between hardphase grains are forced to behave mechanically as the confining material. When increasing the binder layer thickness, the influence from the hardphase will decline, following an exponential relationship. This model has the advantages over current models that it predicts the hardness from data on carbide grain size and volume fraction only, without the need for the laborious carbide contiguity value. It also covers a wider range of carbide grain sizes and binder volume fractions than do the current models. The model has been verified on a very wide range of cemented carbide compositions, covering a hardness interval of 800–2400 Vickers. Throughout this interval, the calculated hardness values agree to within 15% to the measured values. This makes the model a useful tool in the development of new WC–Co grades, for the interpretation of wear results and for estimating grain size and cobalt volume fraction values.

Improvements of the microstructure and erosion resistance of boron carbide with additives

A series of test materials were produced from boron carbide (B4C) powders with additions of either boron in amounts up to 60 wt.%, silicon (4 wt.%) or silicon and silicon carbide (4 wt.% and 30 wt.%, respectively). The powder mixtures were densified by encapsulation hot-isostatic pressing. The test materials where evaluated in dry particle erosion tests with silicon carbide grits. Particular attention was given to the relation between the microstructure and the composition.It was found that boron additions up to 20 wt.%, decreased the average grain size and reduced the porosity of the boron carbide. A material with 60 wt.% boron exhibited very low porosity and supreme resistance to particle erosion. The erosion resistance was also significantly improved by additions of silicon and silicon carbide.The favorable effects of boron, silicon and silicon carbide are discussed in terms of their influence on microstructural parameters, such as grain size, porosity, grain boundaries and reduction of free carbon.

Wear phenomena on WC-based face seal rings

Abstract In order to study wear mechanisms on WC-based face seals, lubricated seal tests have been performed. The tested materials were WC–Co, WC–Ni and a WC–TiC binderless carbide. The tests lasted for 700 h and all seals were still fully functional at the end of the tests. The worn surfaces have been studied with a range of different techniques including focused ion beam and transmission electron microscopy (TEM). All tested seals showed a periodic wear pattern extending perpendicular to the sliding direction. On the metal bonded cemented carbides the pattern included cracks penetrating about 40 μm into the material. In connection to each crack on the WC–Co seals, a damaged subsurface zone with altered structure was found. X-ray analysis of the altered zone revealed the presence of oxides in the vicinity of the cracks as well as an enrichment of Cl in the crack tips. The mechanisms that may have caused the surface damages are discussed.

Grooving wear of single-crystal tungsten carbide

Abstract The anisotropic nature of tungsten carbide (WC) single crystals has been evaluated in single-tip scratch testing and in multiple-tip abrasion. The single-tip grooves were made with a Vickers diamond indenter and the abrasion tests were performed with diamond and silica grits. All tests were performed on both the prism and basal planes of the WC crystals. A polycrystalline binderless carbide (Bl) was also evaluated. Optical surface profilometry was used to estimate the amounts of displaced, removed and ridge-formatted material in the scratch tests and the wear volumes in the abrasion tests. The scratches and wear scars were studied with scanning electron-, atomic force- and light optical microscopy (SEM, AFM, LOM). In situ studies of the scratch process were also performed. Wear debris were analysed with transmission electron microscopy (TEM). The results show that there are differences in both the amount of wear and the wear mechanisms between different crystallographic directions of WC. Depending on the direction of the slip planes in relation to the groove direction, the wear mechanisms change from ductile (grooves parallel to the slip planes) to brittle (grooves perpendicular to the slip planes). It is also shown that WC tends to wear by a formation of angular rod-shaped wear debris with the slip planes as the preferred surface planes.