Robert M. Hooper

Indentation hardness and fracture toughness in single crystal TiC0.96

Abstract Using the indentation technique, both the hardness and fracture toughness anisotropy of a titanium carbide single crystal have been investigated on the (001), (110) and (111) planes. The hardness values varied between 20 and 32 GPa, and the anisotropies were consistent with {{110}} 〈 1 1 ¯ 0 〉 slip. The indentation critical stress intensity factors KIa were in the range 1.5–3.6 MPam1/2. The anisotropy of KIa on (001) and (110) could be explained by a bond-breaking model but not that on (111). The experimental values were not consistent with the predicted ones.

The wear of polycrystalline diamond tools used in the cutting of metal matrix composites

Abstract Polycrystalline diamond (PCD) has been used for some years for the machining of abrasive systems such as aluminium/silicon and glass filled polymeric materials. This paper reports on the machining of the highly abrasive composite system of aluminium reinforced by silicon carbide particles and/or alumina fibres using polycrystalline diamond tools. The results indicate that polycrystalline diamond tooling offers superior performance over carbide, both in wear resistance and the quality of the surface finish produced. Observations of the morphology of the wear scars on the tools indicate that the wear process involves both adhesive wear and of the build up of defects within the diamond particles leading to eventual micro- and macro-fracture in a fatigue-like process.

Friction of metal sliders on toughened zirconia ceramic between 298 and 973 K

Abstract This paper reports the results of an investigation of the coefficients of dynamic friction, in unlubricated conditions, of stainless and medium carbon steels, copper and aluminium sliders on a commercially available ceria stabilized, high toughness zirconia ceramic at temperatures up to 973 K. These sliding materials were selected to provide a range of chemical and physical properties representative of commercially available alloys. Measurements were made using 136° apical angle metal cones against the polished zirconia ceramic in air, using a normal load of 3.9 N. The friction coefficients increased from about 0.1 at room temperature to greater than 0.5 at elevated temperatures. Optical and scanning electron microscopy, with energy-dispersive X-ray analysis, showed that the increase in the friction coefficient with increasing temperature was associated with the transfer of slider material onto the ceramic counterface.

Indentation fracture and soft impresser fatigue in sapphire and polycrystalline alumina

Abstract The soft impresser technique has many advantages compared with conventional diamond pyramid indentation for studying the deformation and fracture of materials that would normally be considered ‘brittle’. In particular, as utilised in this study, it can be used to study the evolution of deformation and crack initiation and propagation under contact fatigue conditions. Alumina, both in its sintered polycrystalline form and as single crystals (sapphire), has been investigated because of its many uses in engineering load-bearing applications. Intially conical tool steel impressers were used to subject the flat ceramic surfaces to mean pressures of 5.2 ± 2.4 GPa for up to 7.5 × 10 6 cycles. Plastic deformation in the polycrystalline alumina was observed after only 10 3 loadings, followed by cracking at the edge of the contact zone and subsequent crack interlinking and radial extension. The sapphire showed no signs of deformation or cracking under these conditions, but did deform and crack after only one cycle when a harder WC/Co impresser was used.

Wear processes in the diamond grinding of zirconia ceramics

Abstract he wear features on a number of test blocks of partially stabilized zirconia used in creep feed grinding tests have been examined using scanning electron microscopy. The morphology of the as-ground surfaces suggests that in addition to the expected abrasive wear component a delamination wear mechanism is also operating.

The measurement of surface contact fatigue and its application to engineering ceramics

Abstract Contact fatigue deformation and fracture in three zirconia ceramics has been induced by pressing 120° hardened silver steel conical tips against flat polished ceramic surfaces. The repeated point contact loading method that has been developed is described. It has been shown that it is possible for a softer material to induce cumulative plastic deformation, and subsequent fracture, in a harder material. The performance of single crystal cubic zirconia, tetragonal toughened zirconia and partially stabilized zirconia have been compared. In all cases, the ground tips of the cones plastically deformed during the initial loading cycle to produce a flattened end which conformed with the zirconia substrate. Cracking arising from fatigue damage under cyclic loads of 19.6 ± 9.8 N was observed to occur in (001) calcia stabilized zirconia much earlier than it appeared in the (111) plane. In both cases, the formation and progression of a ‘perimeter’ crack was visible at the edge of the contact zones as the number of cycles was increased. It has been proposed that the cracking is initiated by a dislocation interaction mechanism. The subsequent crack propagation is primarily conchoidal in form, rather than crystallographically oriented. A tetragonal to monoclinic martensitic transformation occurred in the ceria stabilized zirconia, beneath and adjacent to the contact zone. The transformation zone increased in size as the number of cycles increased. The volume expansion associated with this transition caused granular lifting, first visible around the periphery of the contacting areas, then intergranular fragmentation, followed by spalling of the substrate. The fatigue mechanism observed in the magnesia partially stabilized zirconia was a combination of the two above, resulting in transgranular fracture. No material transfer, i.e. metal onto ceramic, or vice versa, was generally observed until after the fracture zones had become well extended and fragmented.

Point contact surface fatigue in zirconia ceramics

The surface deformation and fragmentation behaviour of three zirconia ceramics have been studied by using unlubricated metallic repeated point contact loading at room temperature to investigate the possibilities of cyclic fatigue effects. All tests were conducted on a purpose built computer-controlled apparatus. The zirconias studied were ceria stabilized tetragonal polycrystalline, magnesia partially stabilized, and single crystal calcia stabilized. 120° steel cones were cyclically loaded against the flat, polished zirconia counterfaces, and the damage was observed and analysed as a function of the number of cycles, up to a total of 5 x 10 5 cycles, for loads of 19.6 ± 9.8 N. The ground tips of the cones plastically deformed during the initial loading cycle to produce a flattened end which conformed with the zirconia counterface. The contact pressures were in the range 4 to 10 GPa. In all cases plastic deformation was observed in the zirconias within, and adjacent to, the contact areas. The degree of plastic deformation increased with increasing number of cycles. After approximately 1 x 10 4 cycles, localized cracking was induced at the peripheries of the contact zones, which gradually increased in extent until after 5 x 10 5 cycles there was extensive fragmentation. No material transfer, i.e. metal onto ceramic, or vice versa, was generally observed until after the surface had become rough as a result of the fracturing.

Surface fatigue in ceria-stabilized polycrystalline tetragonal zirconia between room temperature and 1073 K

An investigation has been conducted to study the fatigue deformation and fracture induced by pressing 120° hardened silver steel conical tips against a flat, polished CeTZP counterface between 293 and 1073 K with cyclic loads of 19.6±9.8 N, for up to 475 000 cycles. The ground tips of the cones plastically deformed during the initial loading cycle to produce a flattened end which conformed with the substrate. This test format has been devised to be comparable to the service conditions that these types of ceramics are likely to experience. At temperatures up to, and including, 673 K, the sequence of events is that a tetragonal to monoclinic transformed zone is formed around the contact zone. This expands as the number of cycles is increased. Much later in the fatigue process, grain lifting occurs at the periphery of the contact zone. This subsequently causes intergranular pitting to progress around the edge of the contact zone followed by extension both away from and into the contact region. At 673 K and above, a small number (3–5) of radial intergranular cracks are formed after a few cycles (≤10) which propagate a substantial distance away from the contact zone.

Repeated point contact loading on Ce-TZP

Abstract The deformation and fragmentation behaviour in a toughened ceria-stabilized zirconia ceramic have been investigated by using unlubricated repeated metallic point contact loading at room temperature to explore the possibilities of cyclic fatigue effects. All tests were conducted on a purpose-designed and built computer-controlled apparatus. 120° hardened silver steel cones were cyclically loaded on the polished Ce-TZP substrate, and the damage was observed and analysed as a function of the number of cycles for loads of 19.6 ± 9.8 N. The ground tips of the cones plastically deformed during the initial loading cycle to produce a flattened end which conformed with the substrate. A tetragonal → monoclinic martensitic transformation occurred in the zirconia beneath, and adjacent to, the contact zone. This transformation zone increased in size as the number of cycles increased, even though there was virtually no change in the diameter of the flattened tip. The expansion associated with this phase change in the zirconia caused granular lifting from the surface, at the edge of the contact zone, that resulted in intergranular fragmentation and spelling of the substrate. The hardness of the substrate in the contact zone increased by approximately 15% after 2×10 5 cycles. Traces of metal transfer onto the ceramic substrate could be observed only at 2×10 5 cycles and above.

Cyclic contact fatigue of CaF2: Stress analysis and experimental results

Abstract Surface fatigue is one of the main wear mechanisms in materials, and as such is of considerable relevance to ceramics, which are widely used because of their wear resistance. The purpose of the present study is to develop a general approach to the analysis of plastic-deformation-induced surface fatigue in ceramics. A model material, CaF 2 , was chosen since it is isostructural with commercially more important ZrO 2 , but readily etch pits to reveal the extent of dislocation activity. The soft impresser technique has been used to cyclically load (001), (110) and (111) CaF 2 planes for up to 10 6 cycles. The dislocation patterns on the surfaces have been explained in terms of the relevant surface stress contours, derived from elasticity theory. It has been determined that a resolved shear stress of 17 ± 1 M Pa is required to initiate dislocation motion for small numbers of cycles, ≤100. However, for high numbers of cycles (≥ 10 5 ), the stress required to continue dislocation motion decreased to 7 ± 1 MPa.