1、 Reference number ISO/TR 26946:2011(E) ISO 2011TECHNICAL REPORT ISO/TR 26946 First edition 2011-11-15 Standard method for porosity measurement of thermally sprayed coatings Mthode normalise de mesure de la porosit des revtements obtenus par projection thermique ISO/TR 26946:2011(E) COPYRIGHT PROTECT
2、ED DOCUMENT ISO 2011 All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or ISOs memb
3、er body in the country of the requester. ISO copyright office Case postale 56 CH-1211 Geneva 20 Tel. + 41 22 749 01 11 Fax + 41 22 749 09 47 E-mail copyrightiso.org Web www.iso.org Published in Switzerland ii ISO 2011 All rights reservedISO/TR 26946:2011(E) ISO 2011 All rights reserved iiiContents P
4、age Foreword iv 1 Scope 1 2 Purpose 1 3 Classification . 1 4 Principle . 1 5 Apparatus . 2 6 Metallographic preparation 2 6.1 General . 2 6.2 Sectioning 2 6.3 Cleaning . 2 6.4 Mounting 3 6.5 Grinding and polishing . 3 7 Metallography procedure 5 8 Presentation of porosity . 6 9 Test report 6 Annex A
5、 (informative) List of t values with different n and values 8 Annex B (informative) Report of an international round robin test on the determination of porosity in plasma sprayed ceramic coatings by using image analysis of metallographically prepared cross sections . 9 B.1 Introduction 9 B.2 Scheme
6、of the round robin test . 9 B.2.1 Participants 9 B.2.2 Sample preparation . 9 B.2.3 Guideline of procedures for porosity measurement . 10 B.3 Results and discussion 11 B.3.1 Initial results 11 B.3.2 Second round test carried out in Japan 12 B.4 Reassessment of the round robin tests by the grinding/p
7、olishing conditions 15 B.5 SEM imaging mode . 19 B.6 Summary 19 ISO/TR 26946:2011(E) iv ISO 2011 All rights reservedForeword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standa
8、rds is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also tak
9、e part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2. The main task of technical committees is
10、 to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote. In exceptional circumstances, when a
11、 technical committee has collected data of a different kind from that which is normally published as an International Standard (“state of the art”, for example), it may decide by a simple majority vote of its participating members to publish a Technical Report. A Technical Report is entirely informa
12、tive in nature and does not have to be reviewed until the data it provides are considered to be no longer valid or useful. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO shall not be held responsible for identifying any or al
13、l such patent rights. ISO/TR 26946 was prepared by Technical Committee ISO/TC 107, Metallic and other inorganic coatings. TECHNICAL REPORT ISO/TR 26946:2011(E) ISO 2011 All rights reserved 1Standard method for porosity measurement of thermally sprayed coatings 1 Scope This Technical Report describes
14、 a method for characterizing the porosity of thermally sprayed coatings by metallographical examination. This method is particularly applicable to oxide coatings, such as Al 2 O 3 , ZrO 2and TiO 2 , produced by plasma spray. It also considers the purposes to test the size, shape and density of pores
15、 for thermally sprayed coatings. 2 Purpose The main purpose of porosity measurement is to determine the quality of a thermally sprayed coating and its freedom from porosity, particularly on those areas of the significant surface that demand a functional requirement. This Technical Report provides a
16、standard process that is suitable for determining the porosity of thermally sprayed coatings, as part of the total quality assurance programme. This Technical Report is also intended to provide a standard way to present the porosity of thermally sprayed coatings. 3 Classification The microstructure
17、of a thermally sprayed ceramic coating is characterized by the existence of various pores, microcracks, splat boundaries and unmelted particles, because of the nature of the process. Although different terms are used, both the pores and the microcracks are volumetric spaces, which are free from coat
18、ing material. The pores can be divided into closed pores, open pores and micropores. Closed pores appear as isolated clustered voids in the coating and have no connection with the surface; open pores appear as the same voids but have a connection with the atmosphere, either directly or from one pore
19、 to another; micropores are either closed or open pores which show dimensions only detectable on a microscopic scale. The difference between pores and microcracks lies mostly in their aspect ratios (ratio of the major axis over the minor axis), so, they are collectively treated as pores. The fractio
20、n of volumetric space covered by the pores in thermally sprayed coatings is defined as porosity. 4 Principle The porosity of thermally sprayed coatings is determined by preparing an area of the inspected coating with a cross-section of high microscopic surface quality, which can be viewed using a li
21、ght microscope or a scanning electron microscope (suggested). A quantitative assessment of the porosity of the inspected coatings is carried out by using an image analysis technique on the microscope. ISO/TR 26946:2011(E) 2 ISO 2011 All rights reserved5 Apparatus The following equipment is necessary
22、 for the porosity measurement of thermally sprayed coatings. 5.1 Cut-off wheels (recommended) or diamond wire or high pressure water-jet cutting equipment, (according to equipment in existence) for sectioning coating samples to a proper size with minimal damage. 5.2 Cleaning apparatus, with ultrason
23、ic equipment. 5.3 Mounting equipment. 5.4 Grinding and polishing equipment, (semi-automated or automated grinding/polishing machines are recommended for consistent reproducibility). 5.5 Scanning electron microscope (recommended) or light microscope, for viewing the inspected sample on a cross-sectio
24、n and obtaining digital images. 5.6 Computer, with analysing software for porosity evaluation on digital images. All equipment should undergo regular maintenance and calibration to assure reliability and repeatability of the measurement. At the same time, all metallographic personnel should have the
25、 proper training to allow them to perform the required functions and analyses. 6 Metallographic preparation 6.1 General Metallographic preparation of thermally sprayed coatings is critical for the porosity results. The requirements for detail and monitoring will vary from system to system, depending
26、 upon the degree of automation in the preparation. The basic steps for the preparation are given in 6.2 to 6.5. 6.2 Sectioning If sectioning is required, two commonly used methods are abrasive wheel cutting or diamond wire cutting. The first one, which is comprised of a diamond or boron nitride saw
27、(more effective in this purpose) that breaks down readily exposed fresh cutting surfaces, is usually best for a wide range of coatings. Sectioning should be done with the cutting force from coating to substrate and minimal clamping pressure on the sample. It will be better to secure the specimen for
28、 sectioning with a soft cushion, such as wood, if possible. The sectioning wheel should be as thin as possible to minimize damage, which must be removed in subsequent steps. Minimum pressure should be applied on the wheel to minimize possible overheating, with cooling by water if possible. The lengt
29、h of the test specimen should be greater than 1 cm. At least five test specimens should be taken from each sample in different positions. 6.3 Cleaning Cleaning is an important step for removing all contaminants from the surface of the specimen. Three methods or any combination are recommended. a) Wa
30、shing samples with soap and water. b) Brushing or soaking samples in solvent, such as acetone/alcohol, followed by application of heat treatment to drive off any internal absorption. c) Cleaning samples by performing an initial/extra vacuum step (if using vacuum impregnation in mounting) to volatili
31、ze any entrapped materials. ISO/TR 26946:2011(E) ISO 2011 All rights reserved 36.4 Mounting For the preparation of polished cross-sections, it is necessary to mount the selected region first so that a flat polished area with minimal edge rounding is obtained. In this case, edge retention can be impr
32、oved by coating the outer surface of the sample with an additional layer during grinding and polishing. Electroless nickel plating or sputtering with a metal layer are commonly used. The mounting procedure/material depends on the following: a) time available for mounting; b) size of porosity and lev
33、el of voids in the coating, and degree of interconnected porosity; c) required viscosity of epoxy for impregnation of porosity is important (the viscosity of the cold-mount epoxy should be medium, especially when porosity in the coating is small and difficult to impregnate); d) hardness of coating v
34、s. mounting material. (The mounting medium should be chosen to allow good edge retention and be of comparable hardness to the coating, in order to minimize difficulties during grinding and polishing.) Cold mounting, which can be assisted by heat, with vacuum impregnation alone and/or pressure impreg
35、nation is recommended. 6.5 Grinding and polishing Generally, grinding and polishing parameters that must be considered/controlled in preparation are listed in Table 1. Additional care must be taken to remove cut-off damage during initial grinding if the sectioning step was used, and avoid over-polis
36、hing with colloidal silica in the final steps of preparation. During grinding, examine the prepared area at each stage to ensure that all the damage from the previous stage has been removed. In the case of polishing, the sample is polished with diamond paste down to 1 m grade, then alumina paste is
37、used with 0,3 m grade. Further polishing with colloidal silica may be required to obtain a scratch-free surface. After polishing, clean the sample in suitable solvents in an ultrasonic bath to remove all polishing debris. It should be noted that porosity evaluation is relatively a complicated proces
38、s and grinding and polishing parameters should be chosen properly for reproducible porosity results. Typical procedures involving both grinding paper and disc formats are shown and suggested in Tables 2 and 3. These procedures will require modification for different coating types and equipment avail
39、able in the specific laboratories. Semi- automatic/automatic machines in conjunction with written procedures that monitor/control critical parameters are recommended, which will result in consistent and reproducible results. The kind and amount of consumables used in the metallographic process are o
40、bviously very critical to the final result. It is important to know the changes in consumable suppliers and these should be considered carefully. The specific trial samples should be run to assure similar performance and results, if changes have to be made to an already established procedure with ne
41、w consumables. Research should always be conducted to judge the preparation by SEM micrographs to confirm that no coarse feature occurs during metallographic preparation which is significant of the presence of pullouts, which inevitably result in deviation. Surface roughness is suggested as a crucia
42、l parameter to evaluate the quality of the preparation, which is connected with porosity range in the inspected coating and should be as low as possible. ISO/TR 26946:2011(E) 4 ISO 2011 All rights reservedTable 1 Grinding and polishing parameters considered/controlled in preparation Parameter Descri
43、ption Pressure Load/mount area Speed Both table and specimen holder Rotation direction Relative rotation of head with respect to table Format Grinding disc vs. grinding papers Polishing: no-nap vs. high-nap clothes Abrasive Diamond, SiC, colloidal silica, Al 2 O 3Orientation How samples are placed i
44、n holder with respect to wheel rotation Frequency How often is lubricant/abrasive applied Kind of lubricant Oil, water, alcohol Quantity of lubricant ml/min Time Processing duration for each step Table 2 Typical procedure with the grinding paper format Surface Grit size Pressure Speed Time Abrasive
45、Lubricant Rotation Grinding papers 180 40 kPa 300 rpm 10 min. (enough papers to flatten specimen and remove damage/edge effects) SiC Usually water Complementary Grinding papers 400,600 and 800 40 kPa 300 rpm 20 min. SiC Usually water Complementary Grinding papers 1000,1200 and 2000 40 kPa 300 rpm 30
46、 min. (usually 2 papers per grit size) SiC Usually water Complementary No- nap cloth Can be in the range of 1 to 6 m diamond 40 kPa 300 rpm Can be in the range of 2 to 4 min. Poly- or mono- crystalline diamond Usually water or alcohol Complementary Higher- nap cloth Usually in the range of 0,3 to 0.
47、5 m 40 kPa 300 rpm Usually 4 to 6 min. Colloidal silica, Al 2 O 3 Usually water or alcohol ComplementaryISO/TR 26946:2011(E) ISO 2011 All rights reserved 5Table 3 Typical procedure with the disc format Surface Grit size Pressure Speed Time Abrasive Lubricant Rotation Fixed diamond or composite disc
48、40 to 60 m 40 kPa 300 rpm 15 min. Poly- or mono- crystalline diamond Usually water Complementary Fixed diamond or composite disc 6 to 9 m 40 kPa 300 rpm 30 min. Poly- or mono- crystalline diamond Usually water Complementary No- nap cloth Can be in the range of 1 to 6 m diamond 40 kPa 300 rpm Can be
49、in the range of 2 to 4 min. Poly- or mono- crystalline diamond Usually water or alcohol Complementary Higher- nap cloth Usually in the range of 0.3 to 0,5 m 40 kPa 300 rpm Usually 4 to 6 min. Colloidal silica, Al 2 O 3Usually water or alcohol Complementary 7 Metallography procedure It should be noted that metallographic examination is only meaningful for the well-prepared sample, as well as appropriate visual and numerical standards based on significant statistical analyses. Pre-coating is necessary with a thin (10 to 20 nm) conducting
