Greg E. Hilmas

Fracture Resistant Super Hard Materials and Hardmetals Composite with Functionally Designed Microstructure

Abstract Polycrystalline diamond and other hard materials are widely used in earth boring, mining, and construction tool applications. Chipping and fracture resistance is often improved by various means at the expense of hardness and wear resistance. This trade-off between wear resistance and chipping resistance hinders the development of hard and super hard materials for many industrial applications. A new approach, characterized as “hard materials composites with functionally designed microstructure” including polycrystalline diamond and cemented tungsten carbide, is discussed. The functionally designed microstructure offers enhanced chipping resistance and toughness without significantly sacrificing wear resistance.

TEM investigation of hot pressed ‐10 vol.%SiC–ZrB2 composite

Abstract Abstract The polytypism of SiC, phase transformation of ZrB2 and the interfaces between SiC and ZrB2 were investigated using high resolution TEM in a hot pressed 10 vol.%SiC‐ZrB2 composite. In most cases, no grain boundary interphases between hexagonal ZrB2 and 6H‐SiC phases were observed with SiC being both inter‐ and intragranular. Occasionally, 6H‐SiC transformed into 3C and 15R and hexagonal ZrB2 transformed into cubic ZrB. High resolution TEM showed no grain boundary interphases in most regions. Energy dispersive X‐ray spectroscopy and electron energy‐loss spectroscopy analyses showed the presence of oxygen throughout the sample. The phase transformation of SiC and ZrB2, and the interphase formation between SiC and ZrB2 grains are discussed.

Selective Laser Sintering of High-density Alumina Ceramic Parts

Manufacturing high temperature ceramic parts with Solid Freeform Fabrication (SFF) technology has been of great research interest lately. Described in this paper is a process based on Selective Laser Sintering (SLS) to fabricate 3-dimensional alumina green parts layer-by-layer, followed by binder burnout and sintering to obtain high-density ceramic parts. This SLS process has been developed to manufacture 3D parts including airplane models, letter bars, etc. from mixtures of alumina powder and stearic acid binder. These mixtures were generated by mixing alumina powder with stearic acid in a ball mill. The average particle size of the alumina particles is 0.26 µm. The SLS process parameters were tuned such that the green parts can be built with reasonably sharp geometrical features and that the sintered parts have good mechanical and physical properties. Proper binder burnout conditions were determined in order that the resultant parts after the binder burnout process have no visible cracks. Alumina ceramic parts were obtained by firing the green parts at 1600°C. The alumina ceramic bars achieved 255 MPa in flexural strength, and the bar relative density achieved is 88%. These results are discussed.

Oxidation of ZrB2-SiC Ultrahigh-Temperature Ceramic Composites in Dissociated Air

The oxidation behavior and surface properties of hot-pressed ZrB 2 -SiC ultrahigh-temperature ceramic composites are investigated under aerothermal heating conditions in the high-temperature, low-pressure partially dissociated airstream of the 1.2 MW Plasmatron facility at the von Karman Institute for Fluid Dynamics. Samples are oxidized at different flow enthalpies for exposure times of up to 20 min at surface temperatures ranging from 1250 to 1575°C. The microstructure and composition of the resulting oxide layers are characterized using electron and optical microscopies, x-ray diffraction, and energy-dispersive x-ray analysis. Comparisons are made with samples oxidized under similar temperature and pressure conditions in a furnace test environment in which atomic oxygen concentrations are negligible. Changes in surface optical properties are documented using spectral reflectance measurements, and effective catalytic efficiencies are estimated using computational fluid dynamics calculations together with measured surface temperatures and heat fluxes.

Temperature Jump Phenomenon During Plasmatron Testing of ZrB2-SiC Ultrahigh-Temperature Ceramics

U LTRAHIGH temperature ceramic (UHTC) materials containing hafnium diboride (HfB2) and zirconium diboride (ZrB2) with a silica former, most commonly silicon carbide (SiC), have been studied extensively over the last decade as materials for leading-edge and control surface components on hypersonic vehicles [1–3]. Such components experience extreme aerothermal heating in chemically aggressive, partially dissociated air environments. Promising aspects of diboride-based UHTC materials include the very high melting points of HfB2 and ZrB2 and their refractory oxides hafnia (HfO2) and zirconia (ZrO2), as well as the high thermal conductivities of HfB2 and ZrB2, which enables efficient heat conduction away from stagnation point regions [4]. Zirconium diboride has some advantages over hafnium diboride as an aerospace material, because it is lighter and less expensive. The oxidation of ZrB2 produces both zirconia and boron oxide (B2O3). Significant oxidation of ZrB2 in atmospheric air begins at about 1050 K. The softening temperature for amorphous B2O3 is in the range of 830–900 K [5]; below about 1500 K, the oxide scale consists of a porousZrO2 network filled with liquidB2O3 that acts as an effective oxygen diffusion barrier [6,7]. However, the vapor pressure of B2O3 increases rapidly with temperature [8], resulting in rapid loss of B2O3 above 1500 K. The residual porous zirconia scale provides little resistance to inward oxygen transport and further oxidation [9,10], making the oxidation resistance of pure ZrB2 insufficient for high-temperature hypersonic vehicle applications. The addition of a silica former to ZrB2 improves its oxidation resistance [11–14]. Compositions containing from 10 to 30% (by volume) SiC have generally been found to be optimal in this regard. The virgin ZrB2-SiC surfaces oxidize through parallel reactions that generate ZrO2,B2O3, and SiO2. LiquidB2O3 mixes with amorphous SiO2 to form a borosilicate glass that seals the ZrO2 scale [15]. With increasing temperature, boron oxide evaporates preferentially from Received 10 August 2011; revision received 13 February 2012; accepted for publication 19 February 2012. Copyright

Optical Emission Spectroscopy During Plasmatron Testing of ZrB2-SiC Ultrahigh-Temperature Ceramic Composites

Optical emission spectroscopy is used to investigate the oxidation of a hot-pressed ZrB 2 -SiC ultrahigh-temperature ceramic composite tested in the 1.2 MW Plasmatron facility at the von Karman Institute for Fluid Dynamics. Time-resolved spectra enable the in situ detection and temporal characterization of electronically excited B, BO, and BO 2 species concentrations directly adjacent to the oxidizing sample surface. The evolution of these boron species correlates well with the transient formation of a complex multilayer oxide scale containing a silica-rich glassy outer layer that limits oxide growth.

Development of a repeatedly adjustable intraocular lens

Purpose: To provide a variable‐focus intraocular lens (IOL) that is able to adjust repeatedly and reversibly. Setting: University of Missouri‐Rolla, Rolla, and Eggleston Adjustable Lens, St. Louis, Missouri, USA. Methods: An adjustable IOL based on a mechanically adjustable design has been developed. Prototypes were fabricated from traditional Perspex® CQ poly(methyl methacrylate) (PMMA) provided off the shelf by a current IOL manufacturer. The prototypes have undergone proof‐of‐concept testing per the requirements of National Institutes of Health grant 1 R41 EY13482–01, with specific attention to the feasibility and safety of continuing development of the lens with in vivo trials. The experimental results presented focus on operational force measurements. Results: Prototype lenses were produced consistently. Operational force measurements indicated that use of the mechanical adjustment mechanism is viable for an adjustable IOL and provides repeated adjustments over time. Turning forces exhibited by the prototypes were low enough to suggest that operation of this adjustable IOL will not damage the capsular bag or ciliary body of the eye, a potential concern in using this design. Biocompatibility and optical quality of the prototype lenses are ensured by use of traditional Perspex CQ PMMA. Conclusions: The mechanically adjustable IOL provides a feasible and promising means of confronting postoperative refractive errors and the changing desires of patients. The viability of this IOL design has been proven, and results suggest that operation of the lens is safe enough to pursue in vivo trials. Evaluation of the biocompatibility of the lens architecture as well as the ultimate goal of noninvasive adjustment using this model are reported in a companion article.

Effect of AlN and Al2O3 additions on the phase relationships and morphology of SiC Part I Compositions and properties

X-ray diffraction was utilized to follow the transformation from β-SiC (3C) to the various α-SiC polytypes in the presence of AlN and Al2O3 additives after hot pressing from 1700 to 2100°C. The 2H- and 6H-polytypes of α-SiC were the predominate polytypes with additions of only AlN or Al2O3, respectively. The amount of 2H- and 6H-polytypes, and subsequently the microstructural morphology of the SiC materials, were found to be controlled by varying the amount of AlN and Al2O3. Improvements in fracture toughness to ∼9 MPa-√m were achieved with flexural strengths ranging from 600 to 900 MPa. These results suggest that accurate control of the polytypic make-up of SiC-based materials, along with their mechanical properties, can be achieved through AlN and Al2O3 additions.

Magnetically adjustable intraocular lens

Purpose: To provide a noninvasive, magnetic adjustment mechanism to the repeatedly and reversibly adjustable, variable‐focus intraocular lens (IOL). Setting: University of Missouri‐Rolla, Rolla, and Eggleston Adjustable Lens, St. Louis, Missouri, USA. Methods: Mechanically adjustable IOLs have been fabricated and tested. Samarium and cobalt rare‐earth magnets have been incorporated into the poly(methyl methacrylate) (PMMA) optic of these adjustable lenses. The stability of samarium and cobalt in the PMMA matrix was examined with leaching studies. Operational force testing of the magnetic optics with emphasis on the rotational forces of adjustment was done. Results: Prototype optics incorporating rare‐earth magnetic inserts were consistently produced. After 32 days in solution, samarium and cobalt concentration reached a maximum of 5 ppm. Operational force measurements indicate that successful adjustments of this lens can be made using external magnetic fields with rotational torques in excess of 0.6 ounce inch produced. Actual lenses were remotely adjusted using magnetic fields. Conclusions: The magnetically adjustable version of this IOL is a viable and promising means of handling the common issues of postoperative refractive errors without the requirement of additional surgery. The repeatedly adjustable mechanism of this lens also holds promise for the developing eyes of pediatric patients and the changing needs of all patients.

Effect of AlN and Al2O3 additions on the phase relationships and morphology of SiC Part II: Microstructural observations

Additions of AlN and Al2O3 to β-SiC hot pressed at 2100°C strongly effect the β- to α-SiC phase transformation and the resultant α-SiC polytypes which are formed. Scanning and transmission electron microscopy were utilized to investigate the microstructural changes occurring in SiC due to these additions and to correlate these observations to their mechanical properties. The results suggest that Al2O3 additions stabilize the formation of the 6H-polytype of α-SiC which grows rapidly into an elongated plate-like morphology, while AlN additions stabilize the 2H-polytype of α-SiC resulting in fine equiaxed 2H-SiC: AlN solid solution grains. It is speculated that the elongated growth of 6H-SiC with Al2O3 additions can be controlled through the simultaneous addition of AlN. The formation of 2H-SiC : AlN solid solution grains inhibits the growth of the 6H-SiC grains since AlN(2H) will not go into solid solution in the SiC(6H) structure, effectively pinning the growth of the 6H-SiC grains.