Rainer Oberacker

Microstructure and mechanical properties of carbon-carbon composites with multilayered pyrocarbon matrix

The effect of matrix microstructure on the mechanical properties of carbon fiber felts infiltrated by isothermal chemical vapor infiltration (CVI) has been studied by optical microscopy, scanning electron microscopy and three-point bending tests. The nonbrittle fracture behavior of the investigated composites is related to multiple crack deflections caused by the interfacial sliding between pyrocarbon layers with a varying texture degree and the delamination microcracking within the highly textured pyrocarbon layer. An increase of the flexural strength is observed by the composite having a multilayered pyrocarbon matrix.

New Hardmetals based on TiB2

Abstract Sintering of TiB 2 -hardmetals with ferrous binders results in the formation of parasitic Fe 2 B which consumes a good part of the ductile binder. Besides Fe 2 B, Ti 2 O 3 is formed during sintering. Thermodynamic considerations dilatometry and X-ray analysis show that the impurities in commercially available TiB 2 -powders are responsible for the development of these phases. It is possible to avoid Fe 2 B by adding metals with a higher affinity to boron than iron. Based on these investigations, TiB 2 -hardmetals with single phase FeCrNi binders or two phase Fe 2 BFeCrNi binders were produced. The binder content was varied between 5 and 20 vol.%. These TiB 2 -hardmetals show better hardness-fracture toughness combinations than conventional WC-Co-hardmetals or Ti(C,N)-cermets and excellent hot hardness. These properties are very promising at the present state of development, and it appears that the full potential of the system has probably not yet been reached. Further progress seems to be possible, when fine grained TiB 2 -powders with low O and C content and little excess boron become available. Such powders are expected to improve sintering behaviour, microstructure and mechanical properties of TiB 2 -hardmetals.

Weibull analysis of ceramics under high stress gradients

Abstract The Weibull parameter m of the strength distribution of ceramics under high stress gradients differs from that for moderate stress gradients. This is shown for contact loading. Bars were loaded by oppositely concentrated forcers via rollers. For most investigated materials, measured contact strengths showed strongly reduced Weibull exponents compared with those from 4-point bending tests. This was the reason for a study, in which the effective volumes and surfaces for the two tests were compared and the influence of the strong stress gradients was considered. Under the assumption of the Weibull theory being valid, the effective surfaces and volumes were computed for the normal stress and the energy release rate criteria. In the second part, it will be shown that the strongly non-homogeneous stresses lead to a reduced Weibull exponent.

Tetragonal-to-monoclinic phase transformation in CeO2-stabilised zirconia under uniaxial loading

Critical stresses for the initiation of the tetragonal-to-monoclinic phase transformation in 9Ce-TZP zirconia materials with five different grain sizes have been studied. The influence of the grain size on the critical transformation stresses has been investigated in multiaxial stress states, namely, in four-point bending, biaxial bending and torsion. It was found that phase transformation occurs as a homogeneous phase transformation with a transformation strain increasing continuously with increasing applied stress and also as an autocatalytic phase transformation with the autocatalytic formation of transformation bands normal to the maximum principal stress. An investigation of the critical transformation stresses under different multiaxial loads in the tensile regime, i.e. with positive hydrostatic stress, showed that both the homogeneous and the autocatalytic transformation do not follow the shear-dilatant criterion investigated in multiaxial compressive testing. The experiments showed that under multiaxial loading the onset of both transformation types can be predicted with the maximum principal stress transformation criterion, with the difference between the critical stresses of both transformation mechanisms strongly decreasing with grain size.

Microstructure and mechanical properties of yttria-stabilised tetragonal zirconia polycrystals (Y-TZP) containing dispersed silicon carbide particles

Abstract The microstructure and mechanical properties of pressureless sintered and subsequently hot isostatic pressed Y-TZP ceramics containing up to 20 vol% of SiC particles were studied. The grain growth of the ZrO 2 matrix was not significantly affected by SiC additions. It was found that the hardness, Youngs modulus and fracture toughness K IC increased, and the bending strength, σ b , decreased with increasing SiC content. The maximum K IC and σ b were 7·8 MPa√m and 849 MPa, respectively. Silicon carbide additions caused increased flaw sizes, and reduced thermal stability of tetragonal zirconia during low-temperature ageing. The dominant toughening mechanism was microcracking. Stress-induced phase transformation played only a secondary role.

Simple Expressions for the Evaluation of Stresses in Sphere-Loaded Disks Under Biaxial Flexure

In testing ceramic materials, sphere-loaded disk tests are of increasing importance. Three different tests including one, four, and six spheres are addressed. For the first two tests, the maximum stress in the disk center was already studied in several papers. In this note, simple analytical relations for the distribution of the tangential stress will be given. The stress distribution in the center region is, for instance, necessary for the determination of effective surface and volume as used for Weibull statistics. For special applications also, stresses across larger distances from the center are desirable. As an example, strength tests on coarse-grained alumina are mentioned briefly which need a fracture mechanics evaluation for interpretation.

Consolidation of Y-TZP/SiC particulate composites by sintering and containerless post-HIPing

Abstract The consolidation of ZrO2-SiC particle composites was studied. Thermodynamical considerations show that the stability region can be extended to higher processing temperatures by adjusting the sintering atmosphere. Densification behaviour, phase composition and microstructure development of such composites during sintering in different gas mixtures out of the Ar-CO-SiO system was studied. A process consisting of pressureless sintering in controlled atmospheres and subsequent containerless post-HIPing was developed which allows full density processing of these materials with up to 30 vol.% SiC particles without any additional sintering additives.

Crack growth data from dynamic tests under contact loading

Abstract Bending and contact strength tests at different loading rates were carried out on a commercial alumina. From the comparison of the two strength tests, it was found that the investigated alumina does not exhibit any subcritical crack growth under contact loading, although a strong subcritical crack growth effect is obvious in bending tests. The different behaviour can be explained as the consequence of mode-II failure accompanied with K I K I >0) in the bending tests.

The use of impedance spectroscopy in damage detection in tetragonal zirconia polycrystals (TZP)

Tetragonal zirconia polycrystals (TZP) were chosen as a model material for evaluating the potential of impedance spectroscopy (IS) as a non-destructive method for the detection of microstructural damage. Cracks were introduced and propagated in these materials either by thermally induced phase transformation or by mechanical loading of pre-cracked specimens. In TZP, microcracking, its propagation and concomitant phase transformations were detected as changes in the capacitive and resistive parts of the electrical impedance at temperatures between 20 and 550°C. This can be correlated with in situ observation by optical microscopy of crack opening and phase transformation zones in four point bending test specimens. Qualitatively all aspects of microcracking described above have been shown to be measurable by IS. Quantitative sensitivity limits in terms of error bands and critical crack lengths are derived from the experimental results and numerical field simulations.

A Residual Stress Intensity Factor Solution for Knoop Indentation Cracks

The residual stress intensity factors at the surface and at the deepest point of the semi-elliptical Knoop indentation crack is determined from the stresses in the damaged zone below the indenter. For this purpose, the weight function approach by Cruse and Besuner was used and wide-range expressions of the geometric function are given. The solution is then applied to a commercial silicon nitride for which all relevant geometrical data are available.