Gerhard Welsch

Precision and surface quality of pure titanium (Ti) castings for dental and biomedical uses are limited because of the high melting temperature and the violent reactivity of Ti with mold materials during casting procedures. This feasibility study evaluates an experimental low‐melting Ti‐Co alloy in term of its microstructure and physical and mechanical properties. Tensile samples of Ti‐12 wt % Co alloy were cast under a protective argon atmosphere. The melting range of the cast samplers was determined. Cast samples were annealed at 1010°C for various time intervals in order to homogenize microstructures. Microstructures were examined by optical and scanning electron microscopy. Tensile strength and microhardness tests were performed and correlated with microstructures resulting from annealing processes. Ti₂Co intermetallic compound coexisted with Ti‐Co solid solution in all samples. The melting range of the alloy was 1062–1088°C, which is 568°C lower than that of Ti. The thickness of the surface oxide scale on cast samples was dramatically reduced to 1–3 μm because of the low‐melting nature of the alloy. Solution treatment at 1010°C for 100 h yields the highest tensile strength. Ultimate tensile strength is measured from 852 to 1240 MPa which is stronger than currently used dental alloys. Microhardness values were ranged from 341 to 488 KHN and elongation was from 1.2 to 1.8%. Both microhardness and percentage elongation are similar to those of dental Co‐Cr alloys. One hundred hours of annealing dissolved dendritic boundaries and transformed the alloy to a more microductitle matrix, however, the intermetallic compound of Ti₂Co remained. © 2013 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 101B: 1419–1427, 2013.

The “average acoustic pulse dispersion length in condensed matter channels” is expanded to show that propagating acoustic pulses may behave as phonons, which can be characterized and used to evaluate the channel through which they have travelled. Employing transducers, pulser/receivers and condensed matter channels, it is possible to generate and detect a ‘harmonic Gaussian’ pulse (i.e. a Gaussian pulse modulated by a phased carrier frequency). Propagating in the channel the pulse expands, its carrier frequency is diminished, but the harmonic Gaussian property stays. This parallels the quantum-mechanics ‘wave packet’ characterization, relating to such entities as ‘photons’ and ‘phonons’. Channel inhomogeneities affect travelling phonons. Application of nonlinear regression analysis to reflected echoes, at the front and back of the channel, uncovers intrinsic signal parameters. A theory provided relating the derived phonon profile change to average scattering length, mean domain size or defect spacing, g _s, a material property. Experimental and analytical results provide g _s estimates for diverse channels: a 10 mm column of glycerine, a 4.8 mm column of Elmer's glue and an 8 mm copper plate. The g _s values agree with length scales in the materials (nanometres to microns) determined by other experiments, e.g. electron microscopy.

This paper is concerned with ‘a probing pulse’ propagating in an inhomogeneous condensed state medium. The pulse expands as it travels. This expansion may be correlated to a material-characteristic dimension – such as the size of a grain or of a free path between scattering sites or of a domain – which is a random variable with a mean denoted as g_s . Moreover, we may ascribe to the propagation a loss constant which is proportional to g_s . The process derives from the fact that, for a relatively large number of scattering sites, it is possible to model the propagation media in terms of a repetitive electrical network. The model supports a Gaussian (bell shape) impulse response and defines g_s . Both theoretical and experimental (ultrasonic) results are provided. The experimental method is relatively fast to perform and the results are highly reproducible.

Fatigue tests are often encumbered by lack of the specimen’s exact length due to mechanical grip effects and by environmental noise factors. Such issues may be mitigated when differential measurements are undertaken between consecutive data states. Our principle-monitoring tool is an ultrasonic pulse-echo process where the primary and secondary echoes along the principle specimen’s axis provide both length and gage length, respectively [D. Hazony, Circuit Systems Signal Process. 14(4), 525–538 (1995)]. The process is also useful for crack detection [I. Mostafa et al., Int. J. Fracture 85, 99–109 (1997)]. Compared with absolute measurements, differential stress-strain monitoring is shown to be highly reproducible and more sensitive providing changes in Young’s modulus and Poisson’s ratio as well as a focus for the detection of an emerging crack. [Work supported by NASA.]

Failures that occur in titanium–ceramic restorations are of concern to clinicians. The formation of poorly adhering oxide on titanium at dental porcelain sintering temperatures causes adherence problems between titanium and porcelain, which is the main limiting factor in the fabrication of titanium–ceramic restorations. To overcome this problem a 1-μm thick Si₃N₄ coating was applied to a titanium surface using a plasma-immersion implantation and deposition method. Such a coating serves as an oxygen diffusion barrier on titanium during the porcelain firings. The protective coating was characterized in the as-deposited condition and after thermal cycling. Cross sections of Ti/Si₃N₄–porcelain interface regions were examined by various electron microscopy methods and by energy dispersive analysis of X-rays to study the Si₃N₄ film's effectiveness in preventing titanium oxidation and in forming a bond with porcelain. The experiments have shown that this Si₃N₄ coating enables significant improvement in Ti–ceramic bonding. © 1999 John Wiley & Sons, Inc. J Biomed Mater Res, 46, 262–270, 1999.

Dental casting requires replication of complex shapes with high fidelity. To achieve this objective, the problem of scale formation on titanium dental castings must be overcome. Scaling occurs readily at high temperatures because of the high reactivity of molten titanium with investment materials. The purpose of this study was to examine the effectiveness of using stable oxide coatings on the mold surface to reduce the interfacial reactions. A traditional phosphate-bonded dental investment, a commercial titanium investment, and an experimental oxide coating were used for the molds. Pure titanium samples were cast, divested, and prepared for scanning electron microscopic and energy dispersive x-ray spectroscopic analyses. Layers of 7- to 10-micron-thick scale were formed on titanium castings during reaction with traditional mold materials. Interface reaction was reduced between the molten titanium and the investment material when yttrium oxide or zirconium oxide coatings were applied to the mold before casting. Less titanium-mold interfacial reactions occurred when protective coatings were used as diffusion barriers for titanium casting. Y2O3-coating oxide particles applied without binder were entrapped in the cast titanium surface layer. Further study of a binder system for Y2O3 coating is needed.