Bernard J. Hockey

Effect of material parameters on the erosion resistance of brittle materials

Erosion data are compared with two theories that have been suggested to explain the erosive behaviour of solids. A dimensional analysis is applied to the variables that are important to erosion, and a multivariate, linear regression analysis is used to fit the data to the dimensional analysis. The results of the linear regression analyses are compared with the two theories in order to evaluate the applicability of these theories to erosion. Although semi-quantitative agreement of the data with the theories is obtained, some discrepancies are apparent. In particular, the dependence of erosion rate on hardness and critical stress intensity factor is greater than predicted by either of the two theories. These discrepancies are attributed primarily to microstructural aspects of erosion that are not modelled by either of the theories.

Atomically sharp cracks in brittle solids: an electron microscopy study

The issue of bond rupture versus microplasticity as an essential mechanism of crack propagation in brittle solids is addressed. A detailed survey of existing theoretical and experimental evidence relating to this issue highlights the need for direct observations of events within the crack-tip “process zone”, at a level approaching 10 nm. Transmission electron microscopy is accordingly used to study arrested cracks about sharp-contact (Vickers indentation and particle impact) sites in Si, Ge, SiC and Al2O3. The nature of the deformation which accommodates the irreversible contact impression is first investigated, in the light of Marshs proposal of an “equivalence” between indentation and crack-tip zone processes. Interfacial and tip regions of the surrounding cracks are then examined for any trace of a plasticity-controlled fracture process. Dislocation-like images are indeed evident at the crack planes, but these are shown to be totally inconsistent with any conventional slip mechanism. The close connection between the dislocation patterns and moiré fringe systems along the cracks points to “lattice mismatch” contrast in association with a partial closure and healing operation at the interface. Analysis of all other details in the crack patterns, e.g. the presence of a crack-front contrast band indicative of a residual strain field and the disposition of interfacial fracture steps relative to the dislocation/moiré system, reinforces this interpretation. It is concluded that the concept of an atomically sharp crack provides a sound basis for the theory of fracture of brittle solids.

The transition from mild to severe wear in alumina during sliding

Abstract The occurrence of a wear transition in alumina during sliding has been investigated experimentally. The results show that a transition from initially mild wear to severe wear occurs abruptly, but only after a defined period of initial wear. The time required for this transition increases with decreasing grain size and decreasing applied load. Examination of wear samples revealed that, during the initial stage, surface material is removed by a plastic grooving process and is accompanied by the accumulation of subsurface dislocations arrays and twins. With continued sliding, internal stresses associated with this accumulating damage eventually results in grain boundary cracking and grain pull-out, which leads to the onset of fracture dominated, severe wear.

Transient creep behaviour of hot isostatically pressed silicon nitride

Transient creep is shown to dominate the high-temperature behaviour of a grade of hot isostatically pressed silicon nitride containing only 4 wt% Y2O3 as a sintering aid. Contributing factors to transient creep are discussed and it is concluded that the most likely cause of longterm transient creep in the present study is intergranular sliding and interlocking of silicon nitride grains. In early stages of creep, devitrification of the intergranular phase, and intergranular flow of that phase may also contribute to the transient creep process. The occurrence of transient creep precluded the determination of an activation energy on the as-received material. However, after creep in the temperature range 1330–1430°C for times exceeding approximately 1100 h, an apparent activation energy of ≈ 1260 kJ mol−1 was measured. It is suggested that the apparent activation energy for creep is determined by the mobility and concentration of diffusing species in the intergranular glassy phase. The time-to-rupture was found to be a power function of the minimum strain rate, independent of applied stress or temperature. Hence, creep-rupture behaviour followed a Monkman-Grant relation. A strain rate exponent of − 1.12 was determined.

Tensile creep of whisker-reinforced silicon nitride

This paper presents a study of the creep and creep rupture behaviour of hot-pressed silicon nitride reinforced with 30 vol% SiC whiskers. The material was tested in both tension and compression at temperatures ranging from 1100 to 1250°C for periods as long as 1000 h. A comparison was made between the creep behaviour of whisker-reinforced and whisker-free silicon nitride. Principal findings were: (i) transient creep due to devitrification of the intergranular phase dominates high-temperature creep behaviour; (ii) at high temperatures and stresses, cavitation at the whisker-silicon nitride interface enhances the creep rate and reduces the lifetime of the silicon nitride composite; (iii) resistance to creep deformation is greater in compression than in tension; (iv) the time to rupture is a power function of the creep rate, so that the temperature and stress dependence of the failure time is determined solely by the temperature and stress dependence of the creep rate; (v) as a consequence of differences in grain morphology and glass composition between whisker-free and whisker-reinforced material, little effect of whisker additions on the creep rate was observed.

Creep and fracture of a vitreous-bonded aluminium oxide

Creep and creep-rupture behaviour of a commercial grade of glass-bonded, 96% aluminum oxide was characterized as a function of temperature and applied stress. The creep data were fitted to the classical empirical relation usually used to describe this phenomenon. The apparent activation enthalpy, ΔH = 926 kJ mol−1, and the stress exponent,n = 4.8, lie at the high end of the range reported for two-phase materials, primarily as a result of structural modifications that occur during creep. A stress-modified Monkman-Grant relationship was fitted to the creep-rupture data to give a stress exponent of −4.2. None of the available theories of creep rupture provided a satisfactory description of the present set of data. Analytical electron microscopy was used to characterize the composition and structure of this material. In the as-received material the intergranular phase was a glass of nearly uniform composition. During high-temperature exposure, devitrification of the glass resulted in the formation of various crystalline phases within the intergranular region of the material. Devitrification depended on both the proximity to the surface, where it was most pronounced, and on the state of stress. In this regard, flexural creep samples exhibited extensive crystallization within the tensile region of the flexural specimens, but little crystallization within the compressive cross-section. From the composition of the retained glass, estimates of the viscosity of the glass at the grain boundaries were made and used, in combination with microstructural information, to compare the creep behaviour with available theories of creep. The results of this paper are consistent with percolation and solution precipitation mechanisms of creep deformation. By contrast, cavitation did not seem to play a major role in the creep deformation process.

Properties of Nanostructured Hydroxyapatite Prepared by a Spray Drying Technique

In previous studies nano sized hydroxyapatite (HA) particles were prepared by solgel or precipitation methods, in which the products were washed by aqueous or non-aqueous liquids to remove impurities or undesired components. The washing is know to modify the surfaces of the cystalline particles. This study evaluated properties of nano HA materials prepared by a spray drying method in which the HA product was not exposed to any liquid after its formation. The spray drying apparatus consisted of a nozzle that sprayed an acidic calcium phosphate solution in the form of a fine mist into a stream of filtered air flowing through a heated glass column. The water and volatile acid were evaporated by the time the mist reached the end of the column, and the fine particles were collected by an electrostatic precipitator. Powder x ray diffraction patterns suggested the material was amorphous, exhibiting a single broad peak at 30.5° 2θ. However, high resolution transmission electron microscopic analysis showed that the particles, some of which were 5 nm in size, exhibited well ordered HA lattice fringes. Small area diffraction patterns were indicative of HA. Fourier transfer infrared spectroscopy showed patterns of typical of HA with small amounts of HPO42−. The thermodynamic solubility product of the nano HA was 3.3 × 10−94 compared to 1 × 10−117 for macro scale crystalline HA. These results showed that a spray drying technique can be used to prepare nanometer sized crystalline HA that have significantly different physicochemical properties than those of its bulk-scale counterpart.

High Pressure Compaction and Sintering of Nano-Size γ- Al2O3 Powder

Abstract The effects of pressure on the compaction and subsequent sintering of nano-size Y- γ-Al2O3 powder were studied. Pressures up to 5 GPa were used to compact the powder in a WC piston/cylinder type die and also in a diamond anvil cell. The green body compacts obtained from both methods of compaction were pressureless sintered at temperatures between 1000°C to 1600°C. Results demonstrated that green body density was enhanced with increased compaction pressure. For compaction pressures less than 3 GPa, microstructures containing significant porosity developed at all sintering temperatures studied and is due to the development of a highly porous or vermicular structure during the y too phase transformation, occurring at temperatures between 1000°C and I I50°C. At compaction pressures greater than 3 GPa, however, the formation of the vermicular structure did not occur and near theoretical densities with grain size = 150 nm were obtained.

Wear transitions in monolithic alumina and zirconia-alumina composites

Abstract The tribological properties of composite materials can be improved by its specially designed microstructure. Four zirconia-alumina composites (ZrO 2 -Al 2 O 3 ) and a monolithic alumina were compared under paraffin oil lubricated wear test to investigate the effect of grain size, the residual stress and the phase transformation on the wear transitions. The tensile stress on the contact surface has been identified as the dominant stress which initiates the fracture wear of brittle materials. Wear transition occurs in the brittle materials when the tensile stress at the contact exceeds the critical value. The effect of zirconia content on the mean grain size of alumina and the wear transition resistance of the materials are presented. The dominant factors for the high wear transition resistance of ZrO 2 -Al 2 0 3 composites are attributed to the fine grains with narrow size distribution within the material, the compressive residual stress on the surface due to phase transformation of zirconia and the low elastic moduli of ZrO 2 -Al 2 0 3 composites. The fine grain of the composites and compressive residual stress improve the wear transition resistance, while the low elastic moduli of the composites increase the contact area and reduce the contact tensile stress. The wear transition mechanism of alumina and ZrO 2 -Al 2 0 3 composites are compared by transmission electronic microscopy. During the tribological contact, ZrO 2 -Al 2 0 3 composites generate dense dislocations and twins within the fine alumina and zirconia grains to absorb input energy, while monolithic alumina generates the transgranular cracks.

Contact wear mechanisms of a dental composite with high filler content

The contact wear behavior of a dental ceramic composite containing 92 wt % silica glass and alumina filler particles in a polymeric resin matrix was examined. Because this composite is used for dental restorations, the tests were conducted under contact conditions that were relevant to those that exist in the mouth. Wear tests were conducted on a pin-on-disk tribometer with water as a lubricant. Results on wear volume as a function of load indicated two distinct regimes of wear. The wear volume increased slightly as the load was increased from 1 to 5 N. As the load was further increased to 10 N, the wear volume increased by one order of magnitude. At loads above 10 N (up to a maximum of 20 N), the wear volume was found to be independent of load. Examination of the wear tracks by SEM revealed that a surface film had formed on the wear tracks at all loads. Examination of these films by TEM showed that the films contained a mixture of small gamma-Al2O3 crystallites and glass particles. FTIR analysis of the adhered films indicated the presence of hydrated forms of silica and alumina, suggesting reaction of filler particles with water. Chemical analysis by ICP-MS of water samples collected after the wear tests confirmed the presence of Al and other elemental constituents of the filler particles. It is proposed that three simultaneous processes occur at the sliding contact: tribochemical reactions and film formation, dissolution of the reacted products, and mechanical removal of the film by microfracture. At low loads, wear occurs primarily by a tribochemical mechanism, i.e., formation and dissolution of the reaction products. At higher loads, wear occurs by a combination of tribochemical processes and mechanical detachment of the surface film.