ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689472

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

Knowledge of the elastic properties of oxide scales has long been recognized as important in the understanding of how oxide scales fail at temperature. This paper examines the available measurement methods capable of determining the elastic properties of oxide scales. In the late 1960s a resonant frequency method was proposed that permitted the oxide modulus to be calculated from measurements of the stiffness of an oxidizing rod and knowledge of scale growth rates. This technique was the first to provide an‘in situ’method of measuring oxide properties at temperature. Recent developments now permit a fully automated measurement of oxide elastic properties on both rod-shaped and rectangular specimen geometries. These new developments are reviewed. Alternative methods that are available to measure the elastic properties of materials at elevated temperatures are examined and their applicability to thein situmeasurement of oxide elastic properties discussed. From this analysis it would appear that transient wave excitation methods offer the potential of rapid measurement of the resonant frequency of an oxidizing samplein situand would be capable of utilizing the analyses developed for resonant frequency to determine the oxide modulus.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689473

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

A mechanical properties microprobe (MPM) based on low-load, depth-sensing submicrometre indentation testing is an extremely powerful instrument for examining surface mechanical behaviour and offers several advantages for determining the elastic and plastic properties of protective oxide scales. Its high spatial and force resolutions allow measurements on thin scales and, for thicker surface oxides, determination of gradients in properties. Under optimal conditions, hardness and modulus values can be determined with an overall accuracy of about 10%. A key factor in analysis of the load-displacement data is an accurate determination of the indenter contact area. Because of this, companion indentations into bulk oxides using the same indenter reduce uncertainties associated with comparing the elastic and plastic properties of such materials with their corresponding scales. Use of a standard MPM is currently limited to near room temperature. More effective depth-sensing indentation testing of oxide scales at submicrometre resolution awaits the development of improved specimen preparation techniques, application of advanced analytical methodologies and elevated-temperature operation.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689474

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

Consideration is given to methods currently used for the determination of tensile and compressive fracture strains of thermally formed protective oxide layers at ambient and elevated temperatures. The methods discussed are four-point bend testing in both tension and compression and direct tensile and compression tests. Detection of failure of the oxide is usually by acoustic emission, but in some cases, where the acoustic signal is too small, direct observation of failure can be made byin situmethods using scanning electron microscopy or acoustic microscopy. Details of the tests are described together with the limitations of the different techniques. An attempt is made to estimate errors and consider the effects of important test variables such as strain rate, temperature and dwell periods.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689475

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

Stresses developed isothermally at temperature in both the oxide layer and metal substrate as a consequence of the oxidation process are known as‘growth stresses’. They seem to be an indigenous feature of all oxidation systems and, consequently, offer a potential threat to the long-term integrity of protective oxide layers. To be in a position to provide a comprehensive prediction of lifetime behaviour, it then becomes necessary to characterize the growth stresses for any particular metal/oxide system of interest. An important part of this process is the measurement of both the sign and magnitude of the stresses produced. This paper surveys current experimental methods of performing such measurements and pays particular attention to the advantages and disadvantages of each. Accordingly, critical descriptions are provided of mechanical methods, where shape changes are analysed to deduce the level of stress, X-ray diffraction techniques and also the emerging application of Raman microscopy.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689476

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

High temperature alloys and coatings rely on the formation of adherent scales to protect against further oxidation, but scale spallation is often problematic. Despite the technical importance of the problem,‘practical adhesion’, which refers to the separation of the oxide from the metal, has mainly been treated qualitatively in the past. Various techniques now exist such that the subject can be assessed in quantitative or semiquantitative terms. Some of the techniques are described in this paper, and their weaknesses and strengths are discussed. The experimental methods addressed here include tensile pulling, microindentation, scratch tests, residual stress induced delamination, laser-induced or shock wave induced spallation, double-cantilever beam bending and several four-point beam bending approaches. To date, there is not a universal, easy test for oxide adhesion measurement that can provide reproducible information on interfacial fracture energy for a variety of oxide/metal systems. Much experimentation is still needed to increase confidence in many of the existing tests, and the fundamental mechanics for some present techniques also require further development.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689477

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

Several experimental techniques are being used to obtain numerical information concerning the spallation of protective oxide scales formed during high temperature corrosion. These data are required both for mechanistic modelling of the spallation process and for the construction of spall databases for engineering design purposes. By far the most universally employed experimental method is gravimetry, involving either continuous or discontinuous measurements. Thin layer activation can monitor spallation and has potential for further exploitation. Acoustic emission and, to a lesser extent, the resonant frequency technique are the principal methods used to detect or monitor cracking which precedes spallation. These experimental procedures are described and assessed, while further technique development requirements are detailed.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689478

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

This paper summarizes some recent work concerning the chemistry of interfaces, particularly impurity segregation to interfaces and displacement reactions which can occur at the metal/oxide interface. Different methods of specimen preparation to examine interfaces include a sputter ion plating technique which simulates oxide spalling and ultrahigh vacuumin situbending and scratching techniques. Data are obtained most commonly from techniques such as Auger electron spectroscopy and secondary ion mass spectrometry. Surveys, profiles and elemental maps provide information regarding the chemical composition of the interface. Emphasis is placed on studies of Cr2O3on Fe-Cr alloys and Fe-Cr-Ni- Nb-stabilized steel and Al2O3, on Fe-Cr-Al andβ-NiAl. The observation of sulphur segregation at the metal/oxide interface is considered in some detail, as well as the role of reactive elements. The research shows that impurity segregation and interfacial displacement reactions are important in defining and understanding the chemistry of interfaces, and in turn are likely to influence mechanical properties such as oxide adhesion and spalling.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689479

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

This paper provides a comparison between hardness and elastic modulus data measured using a mechanical properties microprobe (MPM), the acoustic microscope and two techniques based on resonant frequency to measure the elastic moduli of oxide systems. Measured values for bulk oxides, namely AI2O3and Cr2O3, have been used to compare the various measurement systems. The comparison is then extended to measurements on oxide scales. In general, hardness values measured using the MPM technique agree with reported bulk values, although differences between laboratories have been identified which may be attributable to the position of indentation within the scales. Hardness values for scales are found to be similar to hardness values for the bulk, lying in the range 21–30 GPa for A12O3scales and 18–33 GPa for Cr2O3scales. Young's moduli for recrystallized AI2O3have been measured using the mechanical properties microprobe, acoustic microscopy and resonance methods. Data determined using the MPM technique give the highest values, up to 30% higher than values determined by acoustic microscopy or resonance methods. The last two methods agree well with published data. For chromia, Young's moduli measured using MPM techniques agree well with published data. For oxide scales there is good agreement between the MPM technique and resonance techniques where measurements can be compared. For base metal oxides, elastic moduli data are in the range 151–192 GPa for iron oxides, 205–315 GPa for nickel oxide and 116–163 GPa for cobalt oxide. For alloy systems developing Cr203scales, elastic moduli as determined by the MPM are in the range 327–202 GPa. Data measured using resonance methods either fall into this range or are substantially higher. For alloys that develop a substantial internal oxide network, the values measured using resonance methods may well be double or triple those measured within the outer scale by the MPM technique. This is believed to be due to surface interaction effects, possibly the added stiffness provided by an internal oxide network. The resonance techniques are currently the only methods by which the change in modulus with temperature can be investigated.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689480

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor

摘要:

This paper deals with the stresses generated during high temperature oxidation and their relationship to scale fracture. In the first part, the objective is to differentiate growth stresses from thermal stresses and give evidence for relaxation phenomena. The results obtained for materials which develop NiO, Cr2O3or Al2O3scales indicate that, in most cases, growth stresses in the oxide films are negligible compared to thermal stresses, probably on account of stress relaxation at high temperature. Moreover, the stress sign in the oxide scale is inconsistent with conventional views based on the Pilling-Bedworth ratio, but closely related to the growth mechanism of the oxide: a preponderantly cationic diffusion leads to tensile stresses, whereas a preponderantly anionic diffusion leads to compressive stresses. Thermal stresses are closely related to the differences between the expansion coefficients of the scale and the substrate, but can be modified by parameters which can promote stress relaxation such as the cooling rate, the thickness of the film or the thickness of the substrate. In the second part, the objective is to relate growth stress to scale fracture. Growth stresses in oxide scales may be geometrically induced, of intrinsic nature or caused by epitaxial constraints or phase changes. The first two types in particular may lead to local scale failure whereby the number and size of physical defects determine the failure stresses or strains. While more experimental data on intrinsic growth stresses can now be found in the literature, no data exist for geometrically induced stresses. These can, however, be estimated from scale growth kinetics. The level of the kinetics shows their significance for scale failure, and thus it is recommended that more experimental work be done in this field.

ISSN:0960-3409    

DOI:10.1080/09603409.1994.11689481

出版商:Taylor&Francis    

年代:1994

数据来源: Taylor