SIST ENV 12923-2:2002

SLOVENSKI STANDARD
SIST ENV 12923-2:2002
01-januar-2002
Advanced technical ceramics - Monolithic ceramics - Part 2: Oxidation test
Advanced technical ceramics - Monolithic ceramics - Part 2: Oxidation test
Hochleistungskeramik - Monolithische Keramik - Teil 2: Oxidationsprüfung
Céramiques techniques avancées - Céramiques monolithiques - Partie 2: Essai
d'oxydation
Ta slovenski standard je istoveten z: ENV 12923-2:2001
ICS:
81.060.30 Sodobna keramika Advanced ceramics
SIST ENV 12923-2:2002 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST ENV 12923-2:2002

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SIST ENV 12923-2:2002
EUROPEAN PRESTANDARD
ENV 12923-2
PRÉNORME EUROPÉENNE
EUROPÄISCHE VORNORM
July 2001
ICS 81.060.30
English version
Advanced technical ceramics - Monolithic ceramics - Part 2:
Oxidation test
Céramiques techniques avancées - Céramiques Hochleistungskeramik - Monolithische Keramik - Teil 2:
monolithiques - Partie 2: Détermination de l'oxidation Oxidationsprüfung
This European Prestandard (ENV) was approved by CEN on 4 June 2001 as a prospective standard for provisional application.
The period of validity of this ENV is limited initially to three years. After two years the members of CEN will be requested to submit their
comments, particularly on the question whether the ENV can be converted into a European Standard.
CEN members are required to announce the existence of this ENV in the same way as for an EN and to make the ENV available promptly
at national level in an appropriate form. It is permissible to keep conflicting national standards in force (in parallel to the ENV) until the final
decision about the possible conversion of the ENV into an EN is reached.
CEN members are the national standards bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece,
Iceland, Ireland, Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2001 CEN All rights of exploitation in any form and by any means reserved Ref. No. ENV 12923-2:2001 E
worldwide for CEN national Members.

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SIST ENV 12923-2:2002
ENV 12923-2:2001 (E)
Contents
page
Foreword 3
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1SCOPE 4
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2 NORMATIVE REFERENCES 4
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3 TERMS AND DEFINITIONS 5
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4 BACKGROUND 5
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5 INTERFERENCES 6
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6 APPARATUS 8
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7 TEST PIECES 10
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8 TEST PROCEDURE 11
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9 EXPRESSION OF RESULTS 13
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10 REPORT 14
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Annex A (informative) Interlaboratory evaluation of the test procedure 16
................................
Bibliography 18
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ENV 12923-2:2001 (E)
Foreword
This European Prestandard has been prepared by Technical Committee CEN/TC 184
"Advanced technical ceramics", the secretariat of which is held by BSI.
EN 12923 consists of two parts:
Part 1: General practice for undertaking corrosion tests (ENV)
Part 2: Oxidation test (ENV)
Annex A is informative.
This Prestandard includes a Bibliography.
According to the CEN/CENELEC Internal Regulations, the national standards organizations
of the following countries are bound to announce this European Prestandard: Austria,
Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece, Iceland, Ireland,
Italy, Luxembourg, Netherlands, Norway, Portugal, Spain, Sweden, Switzerland and the
United Kingdom.
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ENV 12923-2:2001 (E)
1SCOPE
This Part of ENV 12923 describes a simple oxidation test for advanced technical ceramics.
The test is designed to give an assessment of the mass and dimensional changes of test pieces
following oxidation at high temperature in an oxidizing atmosphere, and to assess whether
oxidation has a significant effect on the subsequent strength, either at room temperature or at
elevated temperatures.
NOTE 1 This test method does not allow definition of other mechanical performance changes
resulting from high-temperature exposure, such as changes in susceptibility to subcritical crack
growth, creep behaviour, migration of secondary constituents, etc.
NOTE 2 This test method does not cover the additional effects of other corrodents in the ambient
atmosphere, such as salt vapours, reducing or corrosive gases, and other contaminants. This method
also does not cover tests at pressures other than ambient atmospheric pressure.
NOTE 3 An interlaboratory evaluation of the procedure given in this standard is summarised in annex
A.
2 NORMATIVE REFERENCES
This European Prestandard incorporates by dated or undated reference, provisions from other
publications. These normative references are cited at the appropriate places in the text and the
publications are listed hereafter. For dated references, subsequent amendments to or revisions
of any of these publications apply to this European Prestandard only when incorporated in it
by amendment or revision. For undated references, the latest edition of the publication
referred to applies (including amendments).
ENV 820-1 Advanced technical ceramics –Monolithic ceramics –Thermomechanical
properties – Part 1: Determination of flexural strength at elevated
temperature.
EN 843-1 Advanced technical ceramics – Monolithic ceramics – Mechanical properties
at room temperature – Part 1: Determination of flexural strength.
ENV 1006 Advanced technical ceramics – Methods of testing monolithic ceramics –
Guidance on the sampling and selection of test pieces.
EN 60584-1 Thermocouples – Part 1: Reference tables (IEC 60584-1:1995).
EN 60584-2 Thermocouples – Part 2: Tolerances (IEC 60584-2:1982+A1:1989).
EN ISO/IEC 17025 General requirements for the competence of testing and calibration
laboratories (ISO/IEC 17025:1999).
ISO 3611 Micrometer callipers for external measurement.
ISO 4677-1 Atmospheres for conditioning and testing – Determination of relative humidity
– Part 1: Aspirated psychrometer method.
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ISO 4677-2 Atmospheres for conditioning and testing – Determination of relative humidity
– Part 2: Whirling psychrometer method.
ISO 6906 Vernier callipers reading to 0.02 mm.
3 TERMS AND DEFINITIONS
For the purposes of this European Prestandard the following terms and definitions apply.
3.1
oxidation
process of reaction of a ceramic material with oxygen in the surrounding atmosphere,
including any internal reactions as a result of the presence of open porosity or of diffusion of
ions to or from the ceramic surface
3.2
catastrophic oxidation
oxidation of a ceramic material which, under prescribed conditions leads to rapid material
destruction as a result of lack of development of protective surface layers, skins or scales
4 BACKGROUND
Non-oxide ceramic materials, such as those based on silicon nitride, silicon carbide, titanium
diboride and boron nitride, are subject to chemical change when exposed to oxygen in
ambient atmospheres at high temperatures. The changes are generally the substitution of the
non-oxygen nonmetallic species by oxygen, which results in a mass change and the
development of a surface skin of altered composition. In addition, the chemical potentials
involved can cause migration of both metallic and nonmetallic species within the near surface
layer, altering the microstructure of the material. In the case of materials with open porosity,
such as reaction-bonded silicon nitride and some silicon carbides, oxidation will generally
occur through continuous pores which are initially surface connected, although these may
become blocked as oxidation proceeds. There may be a consequent gradient of the extent of
oxidation through the thickness of a sample or component.
The extent of oxidation is controlled by the chemical nature of the material, its homogeneity
and the distribution of any adventitious impurities. Local concentrations of impurities or
pores intersected by the surface of the test piece, for example, may lead to locally enhanced
oxidation and the formation of oxidation pits.
The nature of the external surface of the test piece will change. In some cases a glazed
appearance may result if the oxidation products are glassy in character, such as in the case of
some silicon nitrides and silicon carbides. In others where the surface layer becomes
substantially crystalline, it may be matt in appearance. Such layers may be protective of
further oxidation, or substantially slow the process down if they remain intact. However, in
some cases, the oxidation product may not adhere to the sample, but tend to flake off as a
result of disruptive forces caused by volume changes, phase changes and/or thermal
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expansion mismatches and/or thermal cycling. In extreme cases, the oxidation layer becomes
non-protective, and oxidation becomes catastrophic leading to complete disruption of the
material. This may apply particularly to some silicon nitride products when tested in the
intermediate temperature range, typically 800 °C to 1000 °C.
Such chemical and microstructural changes may lead to significant variations in mechanical
properties. In particular, the nature of strength-determining defects may alter, especially if
they are located at or near the surface of test pieces. Full determination of such changes is
beyond the scope of this Prestandard, which is limited in scope to determination of variations
of mass, dimensions and room temperature or elevated temperature strength. These have
relevance to the screening of general material behaviour, and to the mechanical performance
of components which are heated and cooled, and are expected to maintain mechanical
integrity at ambient temperature.
5 INTERFERENCES
5.1 Mass change
5.1.1 Accurate determination of mass changes relies on minimal contact between the test
piece and the apparatus in which it is housed for the duration of the oxidation test. If strong
reaction occurs between the test piece surface and its support, a significant error in registered
mass change may result.
5.1.2 It is recognised that the oxidation reaction at edges or corners of test-pieces may be
modified at these points, and thus the overall mass change per unit nominal area may not be
representative of flat or gently curved surfaces alone. The effect may be minimal for oxidized
layers of a few micrometres thickness, but becomes increasingly significant for thicker layers.
In materials which oxidize internally as a result of containing open porosity, the calculation of
mass change per unit nominal surface area may be influenced by the cross-sectional shape of
the test piece. Results from this test method may not be directly applicable to the behaviour of
components of different shape. However, they may provide a broad comparison of behaviour
between different materials when using test pieces of the same size and geometry.
5.1.3 Free circulation of the test atmosphere is required. If this is for any reason significantly
reduced by the geometrical arrangement of test pieces and supporting apparatus, rates of
oxidation may be reduced.
5.1.4 It should be noted that oxidation behaviour may be influenced by the moisture and
oxygen contents of the atmosphere. It is important to maintain the agreed atmospheric
conditions for the duration of the test.
5.1.5 Accurate measurement of mass of small test-pieces may be limited to ± 0.05 mg, which
defines also the limits of determining whether oxidation has taken place. For mass changes of
this order of size or smaller it is necessary to consider using large test-piece areas.
5.1.6 The loss of material from the test-piece by spallation or even evaporation may occur. If
spallation is suspected, means should be provided to collect the spallation product from each
test-piece, but this may not always be possible.
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5.2 Strength change
The specification EN 843-1 for flexural strength testing may not be achievable after oxidation
if surfaces become rough or uneven during oxidation. The correct alignment function of the
test jig may be interrupted. Such effects cannot at present be quantified. It is recommended
that the alignment of the test-piece and the test jig is checked after a small pre-load is applied
to the test-piece but before testing to fracture.
If the roughness of the oxide scale causes a problem, attempts can be made to flatten the
immediate surface using a hard wooden or soft metal spatula, the intention being to crush
asperities without damaging the underlying dense adherent oxide layer or the remaining
ceramic material. Any such actions must be reported described in the report.
5.3 Dimensional changes
Measurement of true test piece dimensions after oxidation may be uncertain due to the
formation of rough layers, loose scale or loss of scale. This measurement is appropriate for
materials which retain coherent oxide scales.
External dimensional measurement using a micrometer or travelling microscope may not be
reliable, particularly if the thickness of the oxide layer is less than 0,03 mm, or the
morphology is rough. If oxide layer or reaction zone thickness is required under such
circumstances, consideration should be given to examination of polished cross-sections of the
oxidized test pieces.
5.4 Oxidation period
Oxidation rates may be initially very rapid and slow down as a protective skin develops. A
steady-state condition may take more than 100 h to attain, and this situation may never be
achieved, especially at relatively low temperatures. The oxidation period may therefore have
some uncertainty as a result of the heating and/or cooling time required by the furnace system.
The nominal agreed period is that while at constant temperature. However, the oxidation
behaviour determined by the test may be influenced significantly by the period at lower
temperatures during heating or during cooling. For example, it is recognised that reaction-
bonded silicon nitride oxidizes internally by diffusion into open porosity below about 1200
o
C, but the surface tends to seal at higher temperatures. Such factors need to be taken into
account in the report. In the absence of specific prior knowledge of the test material, and
unless the short-term behaviour is specifically required, it is advisable to heat to the test
temperature quickly and to use test durations of at least 100 h, preferably at least 200 h, at the
required test temperature.
Subject to agreement between parties, this test may be performed under controlled thermal
cycling conditions. The results obtained from such tests may be significantly different from
those under steady oxidation conditions, especially if surface oxide layers undergo cracking
or spalling.
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5.5 Surface quality
The oxidation behaviour of an as-manufactured surface may be different from that of a
machined surface, e.g. as occurs with reaction-bonded silicon nitride. It is important to define
the surface finish of test pieces used for the tests.
6 APPARATUS
6.1 High temperature furnace
6.1.1 Any suitable furnace may be employed for this test. The furnace chamber shall have an
inlet for a sufficient supply of oxidizing gas (see 8.1.3) to ensure that the atmosphere does not
stagnate and become oxygen deficient.
6.1.2 The temperature shall be capable of being raised to that required for testing at a
-1
minimum of 5 °C min , of being controlled to better than ± 5 °C at all oxidation
-1 o
temperatures, and of being cooled at more than 5 °C min to below 800 C.
6.1.3 Before commencing oxidation tests, under the agreed conditions of temperature and
environment, the furnace chamber shall be baked out using the same atmosphere as proposed
for testing and at a temperature at least as high as the intended oxidation test temperature for a
period of at least 10 h to remove contaminants.
6.1.4 If required by the agreed conditions of the test, a flowmeter and/or moisture meter in
accordance with ISO 4677 shall be used to monitor the gas flow conditions.
6.2 Test piece support
The test pieces shall be supported using techniques which minimise contact area, degree of
adhesion and extent of reaction with the test piece (see Figure 1). Preferably this should be
done using point or line contact only. Any contact of the supports with the regions of the test
piece surfaces to be subjected later to loading roller contact in flexural strength testing shall
be avoided. In addition, any contact with the test piece surface between the outer roller
positions on the test piece when flexurally tested shall be avoided. Examples of suitable
support methods include horizontal support on small diameter platinum wires, either
suspended or resting on a clean non-reactive ceramic surface, or the use of a block with
drilled holes no more than 3 mm deep such that the test pieces can stand near vertically with a
minimum of end and edge contact.
NOTE 1 It may be necessary to perform some preliminary assessments to ensure that the supporting
material is sufficiently non-reactive to play an insignificant role in determining mass change.
NOTE 2 For silicon nitride materials, platinum wire or silicon carbide particles are preferred as the
contact material. Alumina should be avoided. For high-additive materials, such as sialons, mullite
may be the most appropriate material. For dense silicon carbide ceramics alumina or silicon carbide
contact materials may be used. Platinum is inappropriate for non-oxide ceramics containing free
metallic species, such as silicon carbide containing silicon.
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NOTE 3 For materials which are suspected of giving oxidation layers which might spall, the test
piece support might usefully incorporate a plate or tray to catch any loose debris. The geometry and
size of the tray should not reduce the circulation or air around the test piece.
(a) (b) (c)
Figure 1 - Examples of support systems for flexural strength test-pieces showing: (a) a
refractory block with appropriate-sized holes in it, suitable for muffle furnaces; (b) a support
system based on tubes and discs with holes, suitable for vertical tube furnaces; (c) a pair of
supported parallel rods spaced near the ends of the test-pieces and with an adequate gap
underneath, suitable for a muffle furnace.
6.3 Oven
o
For drying the test pieces, an electric oven capable of maintaining a temperature of 120 C
±10 °C.
6.4 Chemical balance
A chemical balance with a sensitivity of at least 0,05 mg and having an accuracy of this level
or better.
6.5 Micrometer
A micrometer capable of measuring to the nearest 0,01 mm in accordance with ISO 3611.
6.6 Vernier callipers
Vernier callipers measuring to the nearest 0,02 mm in accordance with ISO 6906.
6.7 Thermocouple
Type R or type S thermocouple in accordance with EN 60584-2.
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6.8 Flexural strength test facility
If required, a flexural test jig operating in accordance with the principles laid down in EN
843-1 for room temperature tests, or ENV 820-1 for high temperature tests, to be used in a
suitable mechanical testing machine.
NOTE Any appropriate test piece size may be used (see 7.), although the standard
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