Roger Morrell

Cordierite glass-ceramics-effect of TiO2 and ZrO2 content on phase sequence during heat treatment

A series of glasses of stoichiometric composition with varying proportions of TiO2 and/or ZrO2 as nucleating agent have been studied to examine the effect of nucleation addition on the sequence of crystallization and transformation to the stable phase, cordierite. It is shown that TiO2 is the most effective nucleating agent and that if large amounts of ZrO2 are substituted for TiO2 cristobalite forms as an intermediate phase and is associated with rapid volume changes and consequently with weak porous products. Substitution of ZrO2 for TiO2 also causes other changes in phase development, especially in the relative proportions of β-quartz solid solution and magnesian petalite produced during the early stages of crystallization. The use of a combination of experimental techniques (dilatometry, DTA, X-ray diffraction and electron microscopy) has proved most effective in studying phase development and the relationship between processing characteristics and composition.

Instrumented indentation test for advanced technical ceramics

The standard for advanced technical ceramics ENV 843-4 of 1995 (Vickers, Knoop and Rockwell superficial hardness tests) was validated within the framework of the CERANORM EC-project. The paper reports on depth sensing hardness measurements done for comparison with the other hardness tests. The instrumented indentation test is a modern technique (recent issue ISO/DIS 14577) that has the potential to take into account the specific response of materials in a much better way. An evaluation has been made to establish whether the instrumented hardness technique is a appropriate method for advanced technical ceramics and offers potential for additional applications.

Hardness testing on advanced technical ceramics

To validate the European standard ENV 843-4 for hardness measurements on ceramics, three classes of ceramic materials, silicon nitride, silicon carbide, and aluminium oxide, involving 19 ceramics in total, were tested using the traditional techniques Vickers (HV1), Knoop (HK2), and superficial Rockwell (HR45N). The use of new ceramic reference blocks certified according to the standards ISO 4547 and ISO 6507-1 for metallic materials was studied. If the hardness response of the tested materials does not involve chipping and cracking the application of high hardness reference blocks for training users to obtain hardness values comparable with the certified HV1 and HK2 values improves the reproducibility from about 10% to 1 to 3%. The scatter between the laboratories is similar to the scatter within the laboratories. The measurement of the indentation geometry on typical commercial ceramic materials can be made only with higher scatter and reduced reproducibility compared with typical metallic materials, which is caused by the stochastic indentation response. For such materials involving chipping and cracking (for instance SiC), the application of reference blocks with well-shaped indentations does not provide improved comparability of results between the laboratories. The actual indentation response of the ceramic material tested must be considered before selecting the appropriate hardness technique and test force. There is no significant difference between the abilities of the hardness techniques HV1, HK2, and HR45N to discriminate sensitively between materials of closely similar properties.

25 year perspective Aspects of strain and strength measurement in miniaturised testing for engineering metals and ceramics

Abstract Testing in the microregime is relevant to many technological issues, including, for example, evaluating the performance of established plant or new components through removal of small scale samples; assessing point to point variation in welded structures; characterising expensive advanced alloys that typically are only available in small quantities; and testing of subsized components. The use of miniature testpieces requires attention to the scale of the underlying microstructure relative to the overall testpiece dimensions, and presents challenges to the measurement of stress and strain in small volumes. The subject is extensive and the emphasis of the current paper is on a discussion of issues associated with the measurement of strength and strain, particularly with the latter assessed using non-contact optical techniques based on digital image correlation. The current article thus concentrates on the testing situation at the intermediate scale (in the range 0·5–5 mm) where testpieces are small compared with the size of engineering components and in the range where representative mechanical behaviour of the underlying microstructure must be considered. Metrology issues are addressed through specific case studies concerning; microuniaxial tests; small punch tests; subsized ceramic strength tests and the microindentation technique. Particular attention is paid to the uncertainties in measurement arising from the use of microscale tests to provide quantifiable levels of confidence in the ability of such tests to discriminate material behaviour.

Electron microscope study of non-stoichiometric titania

The distribution and coexistence of Magnéli phases (TinO2n−1) withn=4, 5 and 6 in hydrogen-reduced TiO2 has been investigated using transmission electron microscopy. The major phase was Ti5O9. Ti6O11 was found in Ti5O9 grains in the form of narrow lamellae (<10 nm wide) epitaxially intergrown on (001) planes. The Ti4O7 phase, however, was never observed as a second phase in Ti5O9 grains but formed single-phase grains. The Ti5O9 phase was twinned on the (011)r planes of the rutile subcell structure and the Ti6O11 lamellae were pinned to these twins. This pinning may be responsible for the hysteresis that occurs during the oxidation-reduction of non-stoichiometric TiO2. We also report observations onin situ oxidation of the line phases.

Edge Flaking - Similarity between Quasistatic Indentation and Impact Mechanisms for Brittle Materials

The process of edge flaking of brittle materials is a significant limitation in design, handling and use of components. Simple quasistatic tests to identify resistance to edge flaking can be based on near-edge indentation and scratching towards an edge, and these produce rankings of materials that broadly correlate with GIc or KIc. However, most edge damage occurs in practice by impact. Using a drop-weight impact tester, edge chipping tests have been performed dynamically on a range of brittle materials, using repeated impact with step-wise height increments until fracture. It has been found that when impact energy rather indentation force is used as the correlative parameter against distance of the impact site from the edge of the test-piece, a similar relationship to that of quasistatic indentation is found. The shapes of edge flakes produced may also similar. This suggests that even when a relatively blunt impactor is used, compared with conventional indenters, the mechanics of the failure are similar. However, the occurrence of ring cracks can lead to unusual flake shapes. It follows that simpler-to-perform quasistatic tests can model the less well-defined dynamic impact situation in terms of testing for the effects of geometry or for comparing performance of different material types.

Influence of internal architectures on the fracture response of LTCC components

Low Temperature Co-fired Ceramics (LTCCs) are layered ceramic based components, which – in recent years - are increasingly used as high precision electronic devices (e.g. mobile and automotive technologies) in highly loaded (temperatures, inertia forces, etc.) environments. They consist of a complex three-dimensional micro-network of metal structures embedded within a glass-ceramic substrate. Even though LTCCs have been used for more than 20 years, there is insufficient understanding of the mechanical loads during processing. In this regard, different types of failure of the end component during service have been reported, coming from different parts within the part. In this work, the influence of the internal architectures in the fracture response of LTCC components during bending has been investigated. Strength has been determined in 10 × 10 mm2 specimens using the ball-on-three-balls test (biaxial loading) and evaluated using Weibull statistics. Fractography of broken specimens has been performed to determine the mode of fracture of the components and the role of the internal architecture in the crack path. Results show strength dependence as a function of the testing position within the part. The influence of the internal architecture and residual stresses is also discussed.

Round robin on grain size measurement for advanced technical ceramics

Abstract The results of a joint CEN/VAMAS round robin on grain size measurement for advanced technical ceramics have been analysed. Twenty-five participants from Europe, USA and Japan employed a line and circle method for determination of the mean linear intercept length, and grid methods for determination of a grain size distribution. The results show that the scatter obtained for measurement of the mean linear intercept length and the grain size distribution is primarily due to the influence of micrograph preparation and interpretation and, to a lesser extent, also due to random positioning of lines, circles and grids. The results for the mean linear intercept length measurement validate the methods described in a proposed CEN standard. The grid methods for grain size distribution measurement seem suitable for implementation in future standards. Micrograph preparation and interpretation should be given special attention, as these factors are of major importance for consistent results.

Mechanical properties of ceramics

Advanced ceramic materials have unique properties that make them key elements of current and future technologies. Not only do some forms of ceramic have advantageous refractoriness, hardness, wear resistance, and resistance to corrosion but others also have what is often described as ‘functional’ properties. For example, the high oxygen ion conductivity of zirconia is exploited for high-temperature fuel cells. Some ceramics with perovskite crystal structure can be switched between electrical conducting and nonconducting conditions, triggered by a phase transformation, a feature commonly used in electric circuit protectors. In other perovskite structure ceramics, a strong piezoelectric effect can be developed, and this is exploited in force measurement, for example, load cells, and for displacement control, such as in positioning devices and even as fast-acting valves for diesel injection in vehicle engines.

Fractography and Fracture Toughness Measurement

Using a variety of advanced ceramic materials, a comparison has been conducted of fracture toughness test methods using the single edge vee-notch beam method and the surface crack in flexure method, the latter restricted to optical fractography. Good agreement has been found between the two methods on materials which were amenable to the SCF method. It has further been shown that the SEVNB method can produce reliable results on materials to which the SCF method is not readily applicable.