A.J. Gant

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.

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.

Fracture in ceramic-reinforced metal matrix composites based on high-speed steel

A series of metal matrix composites based upon M3/2 high-speed steel was produced by a powder metal sintering route. Hard ceramic titanium carbide or niobium carbide additions and a self-lubricant in the form of manganese sulfide, were added as a basis for achieving improved wear resistance and reduced friction. After sintering, the composites were given a full standard high-speed steel heat treatment and subjected to mechanical tests. All three particulate additions had a deleterious effect on three-point bend strength, particularly in the case of MnS addition, mainly due to the ease of initiating cracks at or near to the particulate additions. Bend strengths were further reduced by the simultaneous addition of both MnS and either TiC or NbC especially when a high volume fraction of approximately 25 vol % MnS was added. Single, low volume fraction (≤8%) additions, of TiC, NbC, or MnS, had little effect on fracture toughness and KIC values were comparable to those found in the baseline M3/2 steel. Slight improvements in fracture toughness shown to occur in the 7.74% NbC composites were attributed to energy dissipation caused by the effects of crack branching during crack propagation. Composites with the higher volume fraction additions of MnS and ceramic carbide gave poor fracture toughness by forming MnS/carbide clusters which provided an easy path for crack propagation.

Liquid-phase sintering of high-speed steels

Attempts were made to reduce the sintering temperature required to density two different grades of high-speed steel by the introduction of copper-phosphorus to promote liquid-phase sintering and by adding carbon to lower the solidus temperature. The use of nitrogen-atmosphere sintering in an effort to lower the sintering temperature through absorption of interstitial nitrogen into the steel did not succeed. Copper-phosphorus combined with carbon additions was successful in reducing sintering temperatures by approximately 100 ° C but was more effective in the M3/2 (molybdenum) than in the BT6 (tungsten grade) steels. Densification via a liquid-phase sintering mechanism occurs in a number of stages as different liquid phases are produced by interaction between the copper-phosphorus and the steel. Growth of carbides by solution and reprecipitation due to the transport of alloying elements through these liquid phases is suggested as a major densification process.

Abrasion and toughness property maps for WC/Co hardmetals

Abstract WC/Co hardmetals are generally designed for applications where resistance to wear, particularly abrasion or erosion, needs to be high but without the low toughness often found in very hard materials. Consequently, mapping the relation between wear characteristics and toughness is fundamental to the process of grade selection in commercial hardmetals. In current research at NPL wear resistance has been assessed using the ASTM B611 abrasion test method and toughness through Palmqvist fracture toughness. Both these tests are now well understood, especially the uncertainties associated with the measurements. Both of the two main types of property map are considered: those concerned with properties associated with end use, e.g. abrasion and toughness; and those concerned with relating microstructural parameters to the property of interest i.e. abrasion and its dependence on WC grain size. Comments are made on the effects of WC grain size distribution and mean grain size.

FIB / SEM Determination of Sub-Surface Damage Caused by Micro-Tribology Scratching of WC/Co Hardmetal Samples

WC/Co hardmetals are composite materials that are used in many wear resistant applications. Micro-tribology experiments were carried out on polished samples of WC/Co as model single point abrasion tests to determine the microstructural mechanisms of wear. The mechanisms of wear included plastic deformation and micro-fracture of the WC grains leading to fragmentation of the grains with re-embedment in the binder phase to form layers covering and protecting further damage to the surface.