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Item type:Publication, Access status: without files , Impact of thermal shock on the R-curve and subcritical damage behaviour of flame-sprayed alumina compoundsAn alumina rich Al2O3–ZrO2–TiO2 compound was used for the preparation of self-supporting parts via the rod flame spraying technology. The as-sprayed material was compared to self-supporting parts, fabricated from commercially available alumina rods. For both materials, the flexural strength, the fracture resistance, and the behaviour under subcritical loading was evaluated and compared, either before as well as after cold thermal shock. While the flexural strength and the subcritical damaging of both materials were basically affected through thermal shock in a similar manner, the Al2O3–ZrO2–TiO2 compound showed an outstanding behaviour in terms of its fracture resistance, which was only barely reduced after thermal shock quenching.Publicationtype: Journal Article13 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , On the fracture behaviour of niobium–alumina castablesThe present study addresses the characterisation of niobium–alumina composites in terms of their fracture behaviour. Fine-grained Nb–Al2O3 composites with a niobium content of 60 vol% and produced from different alumina sources (tabular alumina or white fused alumina) were prepared by castable technique. The sintered castables were tested for their sensitivity towards subcritical crack advance, their R-curve behaviour, and the work of fracture. All properties were derived from slow, cyclic four-point flexure tests at room temperature, which allowed for the initialisation and the manual and automatic control of a stable growing crack. Overall, the investigated composites showed measurable R-curve behaviour and were more sensitive towards subcritical crack damage than pure alumina.Publicationtype: Journal Article32 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Using polystyrene spheres to produce thin, porous interlayers in alumina laminatesPorous alumina layers were produced by colloidal processing of alumina with the addition of 15, 30, and 45 vol% polystyrene spheres (PS) as pore formers. Alumina laminates were designed with dense layers alternated with porous interlayers using 45 vol% of PS spheres and sintered at 1350°C. The layers’ thickness ranged from 2 to 15 μm, with a random distribution of pores. The higher volume fraction of pores tends to decrease the alumina average grain size, but does not influence the final size of bulk and surface pores. The obtained values of hardness and Young's modulus for the porous interlayer are ∼30% of the values obtained for the dense layer. Vickers indentations suggested that crack propagation can be opposed by the porous interlayers. However, values of mechanical strength, fracture toughness (KIC), and work of fracture presented no relevant difference compared to a monolithic reference. R-curves presented a slight increase and KIC a decrease due to crack propagation through the porous interlayers. Although no macrodeviations of the crack path were observed in the fractured surfaces, microdeviations were detected in the interlayer regions.Publicationtype: Journal Article58 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Life time prediction of self-supporting flame-sprayed alumina-rich coatingsMain objective of the presented research is the life time prediction of self-supporting flame-sprayed Al O - and Al O -ZrO -TiO -materials under constant load. The characteristic life time and its scatter were derived from stable crack growth tests and Weibull-statistics including the four-point-bending and ball-on-three-balls method. The potential life time was estimated in order to assess room-temperature handling and long term storage of self-supporting flame-sprayed alumina components. In terms of flexural strength, energy release, and subcritical crack growth parameters, distinct differences between both materials were shown. In turn, the characteristic life times only barely deviated from each other. From that the conclusion was drawn that life time performance under constant load application is governed by the characteristic flame-spray microstructure. However, advantages in the flame-spray processing of Al O - ZrO -TiO are still given, attributed to its lower melting temperature.Publicationtype: Journal Article54 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Extended cubic fracture model for porous materials and the dependence of the fracture toughness on the pore size(2020-08); The 3D-model published in the recent paper “A geometric model for the fracture toughness of porous materials” by Jelitto/Schneider (2018) is extended for the case of closed porosity with low density, like in polymer foams. At constant porosity, this model predicts the fracture toughness to be independent of the pore size. On the other hand, the well-known model of Ashby and Gibson yields a slight dependence on the pore size, which is discussed in detail. After investigating the mathematical derivation, it seems that their normalization of the fracture toughness needs to be reconsidered, resulting in a removal of the scale-dependence.Publicationtype: Journal Article88 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Crack propagation behaviour of carbon free and carbon bonded alumina based filter materials(2020-06-01); ; ; ; The room temperature handling of ceramic foam filters for steel melt filtration strongly depends on the mechanical properties of the materials. Against this background, the filter materials Al2O3, Al2O3 – C, and Al2O3–ZrO2 – TiO2 were compared regarding the crack growth exponent n, R-curve behaviour, and energy release during crack growth. Besides, for pure alumina and alumina with zirconia and titania additions, the influence of grain size was addressed. Despite significant differences in Young's Modulus and plateau crack resistance of the R-curve, comparable n were found for Al2O3 - and Al2O3–C -materials. In contrast, the behaviour of the Al2O3–ZrO2 – TiO2 -material differed in many regards, e.g. in the scale of n or in the energy release during crack advance. Moreover, a statistical analysis of the variances in n and the scatter of the evaluated R-curves revealed a reciprocal relation between the scatter in n and the smoothness of the found R-curves.Publicationtype: Journal Article152 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Room temperature R-curve and stable crack growth behaviour of ZrB2–SiC ceramic composites(2019-05-19); ; ; ; R-curve and controlled stable crack growth behaviour of ZrB 2 –17vol.-%SiC and ZrB 2 –45vol.-%SiC ceramic composites was studied on V-notched samples using four-point bending at room temperature. The rising K 1R behaviour was determined as a function of the crack extension Δa with a crack bridging mechanism being dominant in such behaviour. Significant differences in crack growth rates were found within the same composition of ceramics simply as the crack length varied during crack growth processes. These differences are indicative of the significant influence of microstructural parameters of the ceramics on crack propagation. The peculiarities of stress intensity factor K 1 and the crack growth-specific behaviour in ZrB 2 –SiC particulate ceramic composites are discussed.Publicationtype: Journal Article222 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Stable crack propagation in free standing thermal sprayed Al2O3 and Al2 O3 [sbnd]ZrO 2[sbnd]TiO2 coatingsDue to their unique properties and structure, thermally sprayed ceramics are applied in many fields. Their applications range from engineering ceramics to the use in high temperature units, especially since an improved thermal shock performance was observed. The present study presents an analysis of structural and mechanical properties of thermal sprayed alumina-rich coatings at room temperature. Microstructure and phase composition were analysed by scanning electron microscopy and X-ray diffraction. For pure alumina material, high temperature X-ray diffraction was performed additionally and yielded a phase transition within heating to 1200 °C. Micrographs present typically layered and chaotic microstructure, streaked by microcracks. Cyclic loading test-procedures and the resulting load-displacement-curves revealed a pronounced R-curve-behaviour for coatings of pure Al 2 O 3 and Al 2 O 3 [sbnd]ZrO 2 [sbnd]TiO 2 compositions.Publicationtype: Journal Article203 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , R-curve behavior and flexural strength of zirconia-toughened alumina and partially stabilized zirconia composite laminates(2018-08-15); ; ; ; A composite-laminate formed by thick layers ( 320 µm) of zirconia-toughened alumina (ZTA) with thin ( 50 µm) interlayers of zirconia partially stabilized (Y-PSZ) has been fabricated by tape casting and pressureless sintering. Fracture behavior and strength has been investigated and compared to a “monolithic” reference, e.g. a stack of zirconia-toughened alumina (ZTA) without interlayers. The fracture behavior has been analysed using stable crack growth in V-notched specimens loaded in 3-point bending. The ZTA+Y-PSZ composite laminate presented a rising crack resistance with maximum values between 6 and 14 MPa m1/2. In contrast, the “monolithic” ZTA laminate shows a plateau R-curve behavior at 2.7 MPa m1/2. Several toughening mechanisms were identified in the ZTA+Y-PSZ composite laminate, such as, crack arrest/slow down, micro cracking and bifurcation. These toughening mechanisms are most likely caused by high tensile residual stresses that were estimated theoretically.Publicationtype: Journal Article151 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Comparative study of static and cyclic fatigue of ZrB2-SiC ceramic compositesRoom temperature static and cyclic fatigue of ZrB2-32 vol% SiC and ZrB2-45 vol% SiC particulate ceramic composites has been studied. It was established that the presence of grain bridging plays an important role in the lifetime and time dependent mechanical performance of ZrB2-SiC composites. It was also established that the cohesive strength of grain boundaries of the composites was a determining factor if grain bridging would occur during crack growth, as the grain boundaries strength would determine the pathway of the moving crack. Grain bridging was limited in ZrB2-32 vol% SiC leading to the absence of a cyclic fatigue effect, while grain bridging indeed occurred in ZrB2-45 vol%SiC contributing to a cyclic fatigue effect which limits the lifetime of the composite. Such differences were responsible for the occurrence of R-curve behavior in ZrB2-SiC ceramic composites.Publicationtype: Journal Article158