Philip J. Whalen

Structural quality of parts processed by fused deposition

Commercial solid freeform fabrication (SFF) systems, which have been developed for fabrication of wax and polymer parts for form and fit and secondary applications, such as moulds for casting, etc., require further improvements for use in direct processing of structural ceramic and metal parts. Defects, both surface as well as internal, are undesirable in SFF processed ceramic and metal parts for structural and functional applications. Process improvements are needed before any SFF technique can successfully be commercialized for structural ceramic and metal processing. Describes process improvements made in new SFF techniques, called fused deposition of ceramics (FDC) and metals (FDMet), for fabrication of structural and functional ceramic and metal parts. They are based on an existing SFF technique, fused deposition modelling (FDM) and use commercial FDM systems. The current state of SFF technology and commercial FDM systems results in parts with several surface and internal defects which, if not eliminated, severely limit the structural properties of ceramic and metal parts thus produced. Describes systematically, in detail, the nature of these defects and their origins. Discusses several novel strategies for elimination of most of these defects. Shows how some of these strategies have successfully been implemented to result in ceramic parts with structural properties comparable to those obtained in conventionally processed ceramics.

Effect of microstructure on the erosion and impact damage of sintered silicon nitride

The erosion rates and impact damage of two sintered silicon nitride materials with identical compositions but different microstructures were determined as a function of impacting particle (SiC) kinetic energy and temperature (25–1000° C) using a slinger-type erosion apparatus. The coarse-grained silicon nitride had significantly better resistance to impact damage than the fine-grained material. Crack-microstructure interactions were characterized using scanning electron microscopy and showed that crack-bridging was an important toughening mechanism in the coarse-grained material. Post-impact strength data were significantly less than those predicted from the indentation-strength data, due to impact flaws linking up prior to fracture. Consistent with its greater fracture resistance, the erosion rate of the coarse-grained material was less than that of the fine-grained material for erosion at 25 deg, and was independent of erosion temperature.

Yttria migration in Y−TZP during high-temperature annealing

The microstructural and phase changes occurring during the high temperature (1300 to 1550° CO annealing of Y-TZP were studied using X-ray fluorescence, X-ray diffraction, and TEM. Two processes occurred simultaneously involving the diffusion of yttrium. The Y-TZP partitioned into yttria-rich and yttria-poor phases throughout the material, because the material lies in a two-phase two-phase field of the yttria-zirconia phase diagram. The other process involved the segregation of yttrium to the surface, the extent of which was shown to vary with the state of the surface (ground or polished), annealing temperature, and silica content. Migration of yttrium to the surface caused a significant surface composition change (i.e. from 4.7wt% Y2O3 at room temperature to 8.9 wt % Y2O3 at 1550°C for 3 h), resulting in a microstructure and phase composition different from the bulk.

Solid-particle erosion of in situ reinforced Si3N4

Abstract Steady-state solid-particle erosion has been investigated on in situ reinforced Si 3 N 4 and the “equivalent” fine-grained hot-isostatically-pressed Si 3 N 4 whose R-curve behaviors are quite different, having K IC values in the long-crack limit of 8.3 and 5.6 MPa m p 1 2 respectively. Experiments were carried out at 20 °C, using SiC abrasives, whose diameters ranged from 42 to 1035 μm, varying the angle of impact from 15 to 90° and the velocity from 50 to 100 to 150 m s −1 . The erosion rates of the two materials were, within a factor of two, the same, indicating that the long-crack-length-limit toughness is not an indication of erosion resistance, for the range of particle sizes and velocities studied.

Texture on Ground, Fractured, and Aged Y-TZP Surfaces

The phase composition of Y-TZP surfaces has been shown to vary greatly depending on the thermo-mechanical history of the surface. The orientation of these different phases in the surface region is not always random. There is speculation that the alignment of the tetragonal phase before fracturing may play a part in increasing the toughness of these materials. This article deals with an X-ray diffraction analysis of various Y-TZP surfaces with special emphasis on the texture of the different phases. Surfaces which have been ground (and polished), fractured, and aged (200°C) have been examined. In all cases, the monoclinic component that was formed was strongly oriented. The tetragonal phase may or may not be oriented depending on surface treatment. Annealing above the monoclinic-tetragonal transition temperature had little effect on the tetragonal orientation in most cases. Samples fractured at 1000°C have no unusual orientation on the fracture faces.

Transient and Steady-State Erosion of In Situ-Reinforced Si3N4

Relative to most other materials, silicon nitride is very erosion resistant. However, the resulting surface flaws degrade strength - a serious concern for component designers. An in situ reinforced silicon nitride, GS-44, was eroded in a slinger apparatus. Both transient (extremely low level) and steady-state erosion regimes were investigated. Alumina particles with effective average diameters of 140 μm and 63 μm were used at velocities of 50, 100, and 138 m/s. The biaxial tensile strength of the eroded specimens was measured. The strength decreased by about 15 percent after a very small erodent dosage and then remained virtually constant with further erosion.

Creep Recovery Mechanisms

Most studies1–8 in ceramics in which anelastic recovery has been investigated have found recoverable strains less than 10% of the creep strain. Recently, however, Haig et al.1 found as much as 43% of the initial creep strain was recoverable in in situ reinforced silicon nitride and Holmes, et al. (2) found a similar amount of anelastic recovery in fiber reinforced silicon nitride during the first cycle and over 90% of the strain recovered in subsequent cycles. Interest in anelasticity in ceramics is likely to increase since viscoelastic theory predicts large amounts of recovered strain in many two phase composite materials.