Key Engineering Materials Vols. 317-318

Abstract: Thermal shock is a mechanism often leading to failure of ceramic materials that may occur during rapid heating or cooling. These tests were performed in order to compare the thermal shock resistance of ceramic materials by cooling with that of the heating method and hence to evaluate parameters such as thermal shock strength (R1c) and thermal shock fracture toughness (R2c). During the present study, thermal shock resistance of alumina and mullite ceramics was estimated experimentally and theoretically using the thermal shock parameters. The critical thermal stress at the onset of thermal shock fracture was calculated using fracture time, which is measured by an acoustic emission. Results show that thermal shock parameters of alumina specimens decreased with increasing temperature of fracture point. This effect can be attributed to the temperature dependence of the thermal properties. The experimental values of thermal shock parameters evaluated by IRH and WFC techniques were in good agreement at the temperature of fracture point. The thermal shock parameters enabled the definition of a unified thermal shock resistance of ceramics, which is independent of the nature of the testing techniques.

Abstract: The coating of molten silicate glass on a porous carbon substrate was developed, without the formation of cristobalite at the carbon-glass layer interface, in order to improve the steam oxidation and thermal shock resistance. Initially, suitable conditions for coating were assumed from thermodynamic analysis. Based on these calculations, the wettability of the carbon to molten glass was modified by infiltration and pyrolysis of a Si-N precursor, and the coating with glass was carried out under higher N2 partial pressures. As a result, carbon substrates were completely sealed with glass, without the production of cristobalite at the interface, and the glass was infiltrated into the substrate. In contrast, coating with glass at lower N2 partial pressures, such as in Ar, were followed by the formation of cristobalite along with many pores at the interface. The structural changes occurring as a result of variation of the N2 partial pressure during sealing with glass are in good agreement with the thermodynamic analysis. The glass-coated carbon materials, which were fabricated at higher N2 partial pressure, possessed excellent steam oxidation resistance and thermal shock resistance.

Abstract: Sliding wear surfaces of alumina were observed by transmission and scanning electron microscopy. Wear loss was very low (<10-7 mm3/Nm) at room temperature whereas it was high (>10-5 mm3/Nm) at 500oC. In a mild wear regime at room temperature, a layer of very fine particles (10 nm or less) is observed on the wear surface. Beneath the layer, a deformed bulk surface with extensive dislocations is observed. The layer exhibits a diffraction pattern of a meta-stable phase which is dissimilar to the original α-alumina. The layer appears to act as lubrication film at the contact interface to reduce the wear loss. In a severe wear regime at 500oC, a similar structure consisting of a fine-particle layer and deformed bulk surface is observed. However, extensive microcracks are observed in the layer and the bulk underneath that cause further material removal resulting in a high wear rate.

Abstract: The effects of microstructure and composition on the wear properties of Lu sialon ceramics have been studied under dry sliding conditions through block-on-ring wear tests. Microstructural and compositional effects on wear behaviour were studied by producing both equiaxed and elongated α sialons through the incorporation of additional oxides to promote extended liquid formation and grain growth, and by producing α / β composite materials with elongated β grains. The wear response of the materials is discussed in terms of the dominant wear mechanism under different experimental conditions. Under higher loads, where fracture dominates, materials with improved mechanical properties show better wear resistance and both the composite materials and the elongated α sialons showed lower wear rates than the equiaxed materials due to the elongated grain microstructures. Under low normal loads, fracture does not occur and the dominant wear mechanism is thought to be tribochemically assisted wear. Under these conditions, the equiaxed materials had better wear resistance than the composites, and the Lu-α sialon showed an order of magnitude lower wear rate than an equivalent Y-α sialon, thought to be due to better oxidation resistance and improved refractory nature afforded through the use of the smaller radius cation. The elongated Lu-α sialons under these low load conditions showed wear resistance that was to some extent dependent on the composition of the additional liquid phase, with high SiO2 contents leading to higher wear rates.

Abstract: A method is presented for more than doubling the volume yield from an AlPO4 precursor, aluminium chlorophosphate ethanolate, from approximately 20% to 50% by replacing the ethanol ligands with water. It is shown that this increases the volume yield from about 20% to 50%. The precursor is used to make an Al2O3 powder bonded with an amorphous AlPO4 with a Young modulus of 80 GPa and a strength of 60 MPa.

Abstract: Rolling life properties of ceramic bearings in water were studied. In addition, we investigated the influence of ceramics, such as Si3N4, ZrO2 and SiC, on rolling life of bearings in water. All ceramic bearings possess from 15 to 70 times longer rolling life than all AISI440C steel bearings. All Si3N4 and ZrO2 ceramic bearings possess from 3.5 to 5 times longer rolling life than hybrid bearings. Rolling life of all SiC ceramic bearings is approximately the same as hybrid bearings. Rolling life of ceramic bearings in water is mainly due to propagation of cracks, pitting and flaking.

Abstract: Alternate layered composites of Si3N4 layers and layers of Si3N4 with 20 vol% of hBN (Si3N4-BN layers) were fabricated and their tribological properties and thermal conductivities were evaluated. The layered composites were fabricated by alternate stacking of a monolithic Si3N4 layer and a Si3N4-BN layer in the form of a green sheet, followed by hot-pressing or annealing. For comparison, Si3N4 and Si3N4 with 10% hBN were fabricated by hot-pressing powder mixtures. Tribological properties were evaluated on the side plane of the composites by a block-on-ring test method under a dry sliding condition and thermal conductivities were evaluated. The layered composites were found to have lower friction coefficients and higher wear resistance than to simple composites. Moreover, for the layered composite with an aligned β-Si3N4, the friction coefficient on the plane composed of faceted hexagonal grains was lower. The layered composite with an aligned β-Si3N4 in the Si3N4 layer, fabricated by annealing for 72 h indicated both a low friction coefficient (0.28) and high thermal conductivity (130 W/m
K).

Abstract: The microstructure and mechanical properties including wear resistance of Al2O3-based nanocomposites with 5 vol% of Cu and Ni-Co dispersions were investigated. Al2O3/Cu and Al2O3/ Ni-Co nanocomposites were fabricated by hydrogen reduction and sintering process using metal oxide and metal nitrates. The composites showed homogeneous microstructures with nano-sized metal dispersions and enhanced fracture strength and toughness compared with monolithic Al2O3. In particular, high toughness and hardness were measured for the Al2O3/Ni-Co nanocomposite consolidated by PECS. A minimum wear coefficient of 2.33 x 10-5 mm3/Nm was obtained for the Al2O3/Ni-Co nanocomposite, while the monolithic Al2O3 showed a value of 2.0 x 10-5 mm3/Nm. Wear behavior is discussed in terms of microstructure and mechanical properties of the nanocomposites.

Abstract: In the surface machining of brittle materials, there exists a transition from brittle to ductile modes when the depth of cut is reduced below a critical size using ultrafine abrasive grains. Vitrified grinding wheels containing ultrafine abrasives in the sub-micrometer to nanometer range were fabricated by mechanochemically milling nanoabrasive particles and subsequent viscous sintering of abrasive-binder composites. The grinding characteristics of the nanoabrasive grinding wheels were evaluated for the fine grinding of silicon wafers in terms of a variety of variables. Preliminary wafer grinding results are presented on material removal rate and surface quality of silicon wafers.

Abstract: The sliding wear property of plasma sprayed TiO2 coatings with porosity of 4.2% and 5.4% mating against silicon nitride ball were comparatively investigated with a reciprocating tribometer under dry conditions. The results indicate that the wear resistance of the TiO2 coating with lower porosity is superior, which is attributed to its homogeneous microstructure and improved microhardness. The morphologies of the wear tracks of the two TiO2 coatings, as well as wear debris reveal the pore is the main location wear occur. The coefficients of friction of the two TiO2 coatings decrease with the increase of applied load, while the wear rates increase, which is attributed to the difference of the dependence of microhardness on load between the coating and the Si3N4 ball. No apparent dependence of wear resistance on sliding speed was found on these two coatings.