W. Mark Rainforth

The sliding wear of ceramics

The micromechanical and microchemical processes which take place during the friction of ceramics are not fully understood. The occurrence of surface plasticity and microfracture on a sub-grain size scale have been suggested but no clear experimental evidence has been provided. Tribochemical wear mechanisms are now considered to be critical to the wear performance of a ceramic. However, the mechanisms of tribochemical interaction remain unclear. This paper presents a detailed study of the wear of zirconia toughened ceramics against metal and ceramic counterfaces in the regime that the above mechanisms would be expected to operate. No evidence of surface plasticity or microfracture was obtained. The surface contact was found to be largely elastic with minimal transformation of the tetragonal to monoclinic phase. The dominant wear mechanism was tribochemical as a result of dissolution of the ceramic surface into a metallic oxide transfer film for the metal counterface, and the formation of an amorphous surface hydrate against a ceramic counterface. In both instances, it is considered that the tribochemical reaction dictates the minimum wear rate achievable and therefore particular attention should be paid to modification of the ceramic composition to optimise the surface chemistry to promote the optimum hydrate properties.

Dynamic surface microstructural changes during tribological contact that determine the wear behaviour of hip prostheses: metals and ceramics.

It is often the dynamic microstructural changes induced by tribological contact that determine whether or not a material exhibits good wear resistance. It is well known that the mechanical properties of a surface are significantly different from the bulk, an effect amplified by wear induced plastic deformation and electrochemical effects. Despite the importance of these dynamic microstructural changes, there remains little quantitative understanding of how the surface microstructure changes during tribo-contact, and how this modifies the surface mechanical properties and chemical activity. This contribution will focus on key total hip arthroplasty materials, specifically CoCrMo alloys, third and fourth generation alumina/zirconia toughened alumina. High resolution techniques have been used to characterise the wear induced microstructural changes for both in vivo and in vitro samples, which has provided new insight into the wear mechanisms. The results are discussed in detail, in particular, how they inform future materials development for this important application.

Dense zirconia-SiC platelet composites made by pressureless sintering and hot pressing

Abstract 2.6 mol% Y-TZP composites with up to 30 vol% SiC platelet additions were fabricated by hot pressing and pressureless sintering to densities above 93%. Borosilicate glass additions aided densification, reduced the thermal mismatch stress between the matrix and the platelets, and reduced grain growth rates in the mildly reducing atmosphere employed. Strength was reduced (up to 50% in hot-pressed material and 60% in pressurelesss-intered composites) by the addition of up to 20 vol% platelets as a result of increased flaw size and tensile thermal mismatch stresses in the matrix. With a matrix grain size of 0.71 μm, platelets increased toughness (from 7.0 to 8.3 MPa √m for a 30 vol% addition) while the amount of transformation decreased from 33 to 22%, demonstrating that mechanisms such as crack deflection were present. Conversely, where the composite was processed to give a matrix grain size which optimised transformation toughening (≥0.85 μm), the addition of the platelets led to a reduction in toughness (from 14.0 to 8.8 M Pa √m for a 30 vol% addition). The volume fraction tetragonal which transformed during fracture to monoclinic zirconia decreased with platelet addition, for all processing conditions. The composite toughness was always lower than the maximum in the monolith arising from transformation toughening alone, due to a low fracture stress in the composite and modulus load transfer, both of which reduce the transformation zone size.

Stabilisation of Fe2O3-rich Perovskite Nanophase in Epitaxial Rare-earth Doped BiFeO3 Films

Researchers have demonstrated that BiFeO3 exhibits ferroelectric hysteresis but none have shown a strong ferromagnetic response in either bulk or thin film without significant structural or compositional modification. When remanent magnetisations are observed in BiFeO3 based thin films, iron oxide second phases are often detected. Using aberration-corrected scanning transmission electron microscopy, atomic resolution electron energy loss spectrum-mapping and quantitative energy dispersive X-ray spectroscopy analysis, we reveal the existence of a new Fe2O3-rich perovskite nanophase, with an approximate formula (Fe0.6Bi0.25Nd0.15)3+ Fe3+O3, formed within epitaxial Ti and Nd doped BiFeO3 perovskite films grown by pulsed laser deposition. The incorporation of Nd and Bi ions on the A-site and coherent growth with the matrix stabilise the Fe2O3-rich perovskite phase and preliminary density functional theory calculations suggest that it should have a ferrimagnetic response. Perovskite-structured Fe2O3 has been reported previously but never conclusively proven when fabricated at high-pressure high-temperature. This work suggests the incorporation of large A-site species may help stabilise perovskite-structured Fe2O3. This finding is therefore significant not only to the thin film but also to the high-pressure community.

3D analysis of thermal and stress evolution during laser cladding of bioactive glass coatings.

Thermal and strain-stress transient fields during laser cladding of bioactive glass coatings on the Ti6Al4V alloy basement were numerically calculated and analysed. Conditions leading to micro-cracking susceptibility of the coating have been investigated using the finite element based modelling supported by experimental results of microscopic investigation of the sample coatings. Consecutive temperature and stress peaks are developed within the cladded material as a result of the laser beam moving along the complex trajectory, which can lead to micro-cracking. The preheated to 500°C base plate allowed for decrease of the laser power and lowering of the cooling speed between the consecutive temperature peaks contributing in such way to achievement of lower cracking susceptibility. The cooling rate during cladding of the second and the third layer was lower than during cladding of the first one, in such way, contributing towards improvement of cracking resistance of the subsequent layers due to progressive accumulation of heat over the process.

Strain-mediated converse magnetoelectric coupling strength manipulation by a thin titanium layer

The manipulation of the strain-mediated magnetoelectric (ME) coupling strength is investigated by inserting a thin Ti layer (0–10 nm) between a 50 nm Co50Fe50 layer and a (011) oriented lead magnesium niobate-lead titanate (PMN-PT) substrate. A record high remanence ratio (Mr/Ms) tunability of 100% has been demonstrated in the 50 nm CoFe/8 nm Ti/PMN-PT heterostructure, when a total in-plane piezoelectric strain of −1821 ppm was applied at an electric field (E-field) of 16 kV/cm. The ME coupling strength is gradually optimized as the Ti layer thickness increases. Magnetic energy calculation showed that with increasing Ti layer thickness the uniaxial magnetic anisotropy energy (Euni) was reduced from 43 ± 1 kJ/m3 to 29.8 ± 1 kJ/m3. The reduction of Euni makes the strain effect dominant in the total magnetic energy, thus gives an obvious enhanced ME coupling strength.

Characterization of worn alumina hip replacement prostheses.

Alumina hip replacement prostheses have been analyzed following in vitro simulated microseparation. The worn surfaces of the alumina acetabular cup and femoral head were investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM) which identified four different wear zones. Focused ion beam (FIB) cross-sectioning was used to section the worn surface and produce 3D reconstructions of the subsurface damage. This showed that the subsurface fracture was predominantly intergranular, with occasional intragranular fracture. Significantly, in all cases, fracture was restricted to the outer layer of grains. The wear mechanisms leading to the generation of the stripe wear region on the worn alumina hip prostheses are proposed and microseparation is believed to play a critical role.

Spinel–rock salt transformation in LiCoMnO4−δ

The transformation on heating LiCoMnO4, with a spinel structure, to LiCoMnO3, with a cation-disordered rock salt structure, accompanied by loss of 25% of the oxygen, has been followed using a combination of diffraction, microscopy and spectroscopy techniques. The transformation does not proceed by a topotactic mechanism, even though the spinel and rock salt phases have a similar, cubic close-packed oxygen sublattice. Instead, the transformation passes through two stages involving, first, precipitation of Li2MnO3, leaving behind a Li-deficient, Co-rich non-stoichiometric spinel and, second, rehomogenization of the two-phase assemblage, accompanied by additional oxygen loss, to give the homogeneous rock salt final product; a combination of electron energy loss spectroscopy and X-ray absorption near edge structure analyses showed oxidation states of Co2+ and Mn3+ in LiCoMnO3. Subsolidus phase diagram determination of the Li2O-CoOx-MnOy system has established the compositional extent of spinel solid solutions at approximately 500°C.

On the structure and oxidation mechanisms in nanoscale hard coatings

Thin-film structures consisting of alternating nanoscale multilayers show substantial hardness increases compared with monolithic coatings of the constituent materials. Coatings, such as TiAlN/VN are deposited using PVD with individual coating bi-layer thickness of ~3nm. Since TiAlN and VN are isostructural and mutually soluble, mixing of the two layers during deposition is expected, which will inevitably affect properties. Energy filtered TEM using a field emission gun source allowed important information on layer structure, but failed to reveal details 600°C, a duplex oxide structure was formed, with several phases observed, including V2O5, TiO2 and AlVO4, with V2O5 being the dominant oxide at the outer layer at 638°C. These coatings exhibit low friction in dry sliding which is believed to arise from the inherently low friction of V2O5. Focused ion beam studies of wear tests at 630°C confirmed that the contact surface comprised small (~50nm), equiaxed and largely agglomerated V2O5 crystals, confirming the hypothesis.

Electric field-controlled magnetization in bilayered magnetic films for magnetoelectric memory

Bilayered magnetic films (Co50Fe50 (CoFe)/Metglas) were RF sputtered on both (001)-oriented and (011)-oriented PMN-PT (lead magnesium niobate-lead titanate) substrates. Electric field-controlled magnetization changes were observed in all these samples: 65 nm CoFe/24 nm Metglas/(001) PMN-PT, 65 nm CoFe/24 nm Metglas/(011) PMN-PT, and 30 nm CoFe/12 nm Metglas/(011) PMN-PT. The maximum magnetic remanence ratio change (ΔMr/Ms) was 46% for CoFe/Metglas/(001) PMN-PT. In this heterostructure, the electric-field created two new non-volatile switchable remanence states and the as-grown remanence state was altered permanently. High-resolution transmission electron microscopy images show a sharp and smooth interface between Metglas and substrate and conversely a rougher interface was observed between Metglas and CoFe films. In the 30 nm CoFe/12 nm Metglas/(011) PMN-PT sample, a large ΔMr/Ms of 80% along the [100] direction was measured, while the ΔMr/Ms along the [01-1] direction was 60% at the applied electric fiel...