1、UDC 629.7.018 : 620.179 : 629.7.023.22 M. 40, February, 1972 INCORPORATED BY ROYAL CHARTER Tekphone: 01429 9ooo BWTXSH STANDARDS UVSTITUTION 2 PARK STREET, LONDON W1A 2BS Tela: 266933 BRITISH STANDARD : AEROSPACE SERIES METHODS FOR MEASURING COATING THICKNESS BY NON-DESTRUCTIVE TESTING CONTENTS Fore
2、word METHODS 1. Scope 2. Terms and definitions 3. Classification of coatings and substrates 4. Underlying principles and applications of methods of measurement 5. General limitations 6. Safety precautions 7. Approvals 8. Testing procedures 9. Calibration standards Page 1 2 2 2 APPENDIX A. Applicable
3、 methods of non-destructive testing 8 FOREWORD This British Standard, prepared under the authority of the Aerospace Industry Standards Committee, is one of a series for the non-destructive testing of aerospace materials, components and structures. Other standards in this series cover radiographic, m
4、agnetic particle, etch and penetrant inspection methods. For the purposes of this standard, combinations of coating and substrate are classified in association with the available methods of measuring coating thickness, and the basic princi- ples of the methods of measurement by means of non-destruct
5、ive testing techniques are described together with the procedures for their application. The standard is not intended to be used as a level of acceptance or rejecticn as both of these aspects should be covered in an appropriate application standard or be agreed between the interested partie s. metho
6、ds., being complementary to one another, should always be considered in relationship to testing as a whole. When non-destructive testing methods or techniques are specified for measuring coating thickness, the most suitable method and amount of inspection compatible with the ultimate application for
7、 the product should be employed. Bearing in mind that the characteristics of indi- vidual types of equipment can vary, it is also essential for the operator to be fully conversant with the equipment used. This standard makes reference to the following: BS 204. Glossary of terms used in telecommunica
8、- tion (including radio) and electronics. BS 205. Glossary of terms used in electrical engine er ing. BS 3683. Glossary of terms used in non-destructive testing. BS 4758. Electroplated coatings of nickel for (ir 3 I I I- - _ BSI M*40 72 lb24bb9 O099393 3 W I M. 40, February, 1972 METHODS 1. SCOPE 1.
9、1 This British Standard is concerned with methods for the measurement by non-destructive means of the thickness of single coatings, such as paint or plating on metallic substrates, and applies both to the inspection of components during their manufacturing stages and also when subsequently in servic
10、e. 1.2 While one of the methods described can be applied to the estimation of the thickness of anodic coatings on aluminium alloys, the general measurement of the depth of conversion coatings, of corrosion, of case hardening and of decarburiza- tim is not considered, 1.3 The standard does not specif
11、y checks on the physical condition of the coatings, e.g., on their surface texture, continuity or adhesion, and it should be borne in mind that variation in these factors may affect die accuracy of thickness mea sur emen t. 2. TERMS AND DEFINITIONS 2.1 For the purposes of this British Standard, the
12、terms and definitions given in BS 3683 apply. Attention is also drawn to BS 204 and BS 205. 2.2 Unless otherwise stated, the terms conducting and magnetic refer specifically to electrically conducting and magnetic electrically conducting properties of materials respectively. 2.3 In addition, by agre
13、ement means that specific agreement has been reached between the purchaser and the supplier, and any inspecting authority concerned. 3. CLASSIFICATION OF COATINGS AND SUBSTRATES 3.1 The applicability of the available methods of measurement by means of non-destructive testing depends on the particula
14、r combination of coating and subs trate under consideration. These com- binations, together with examples given in paren- theses, include the following: (1) Non-conducting coatings on non-magnetic conducting subs trates. (Paint or plas tics on aluminium alloys.) (2) Non-conducting coatings on magnet
15、ic sub- s trates. (Paint or pas tics on s teel.) (3) Non-magnetic conducting coatings on non- magnetic conducting subs trates. (Chromium on bras s.) (4) Non-magnetic coatings on non-conducting substrates. (Chromiuni on plastics.) (5) Non-magnetic conducting coatings on magnetic substrates. (Chromium
16、 on steel.) (6) Magnetic coatings on non-magnetic conduct- ing substrates. (Nickel on stainless steel or brass.) 3 I (7) Magnetic coatings on magnetic substrates, (Nickel on steel.) (8) Magnetic coatings on non-conducting sub- strates. (Nickel on plastics.) 3.2 Attention is drawn to Appendix A in wh
17、ich a convenient means of identifying the methods which can be applied to any particular coating-subs trate combination has been tabulated. Although a number of methods are listed for each general coating- substrate combination, it should not be inferred that each method will necessarily be suitable
18、 for every material combination eligible for inclusion in that particular coating-subs trate group. 4. UNDERLYING PRINCIPLES AND APPLICATIONS OF METHODS OF MEASUREMENT 4.1 Eddy current method 4.1.1 The factors which govern the effective reactance and resistance of the test coil in eddy current inspe
19、ction include: (1) the size and shape of the test coil, (2) the size and shape of the workpiece, (3) the frequency of the alternating current in (4) the electrical conductivity, magnetic perme- (5) temperature, (6) the coupling or separation between the coil and the surface of the workpiece. 4.1.2 I
20、f the influence of the other factors remains sensibly constant, the last mentioned effect can be used to measure the thickness of non-conducting coatings on non-magnetic or magnetic substrates, If the thicknesses and relative electrical conducti- vities of the coating and substrate are suitable the
21、method may also be applied to the measurement of non-magnetic conducting coatings on a non- magnetic conducting base. 4.2 Magnetic attraction method 4.2.1 In the magnetic attraction method, one pole of a permanent magnet is placed in contact with non-magnetic coatings on magnetic subs trates. The no
22、rmally applied force required to pull off the magnet becomes a measure of the coating thick- ness. 4.2.2 Its applications include the measurement of paint and plating on steel and also, of thin magnetic coatings on a non-metalic backing, 4.3 Magnetic reluctance method 4.3.1 By the magnetic reluctanc
23、e method, a non- magnetic coating on a magnetic substrate b6comes a gap in a magnetic circuit and its thickness is estimated in terms of &e reluctance which it thus introduces. The magnetic circuit may be achieved by either a permanent magnet or an electremagnet. the test coil, ability and homogenei
24、ty of the workpiece, 4.3.2 In one variant, the flux associated with a permanent magnet passes partly through the work- piece by way of tlie coating and partly through an auxiliary flux path shunting the workpiece. The latter includes a simple form of fluxmeter which gives readings which may be relat
25、ed to the varying reluctance of coatings of different thichiesses, including the measurement of thin magnetic coat- ings on non-magnetic materials. 4.4 Radiation back scatter method 4.4.1 In applying the radiation back scatter method, radiation (beta-particles, x-rays, bremsstrahlung, etc.) from a s
26、uitable source is directed at the cpat- ing under test. The indications of a suitably disposed radiation detector become a measure of tlie back scatter either arising from the coating or arising froni the substrate and attenuated by the coating and, hence, of its thickness. 4.4.2 mie principle may b
27、e used to measure the thickness of coatings on substrates when tlie thicknesses of both fall between certain limits and when there is a suitable difference between the atomic numbers of tlie materials concerned. 4.5 Thermo-electric inethod 4.5.1 In one application of tlie thermo-electric iiietliod,
28、a metallic probe maintained at a controlled, above ambient temperature is placed in contact with the coating to be measured. The resulting rise in temperature at tlie junction between the coating and the substrate depends on tlie thickness of the coating atid is sensed in terms of the electrical pot
29、ential established between the dis- similar metals, i.e. Seebeck effect. 4.5.2 This nie tiiod lias found particular application in the measurement of magnetic coatings on a magnetic substrate. 4. GENERAL LIMITATIONS 5.1 Characteristics of methods 5.1.1 Non-destructive methods of coating thickness me
30、asurement are applicable only to niateriais wliicli possess the particular property or properties on which such methods are based. Furthermore, if both the coating and the substrate exhibit these properties, they must of necessity do so to a consistently different degree. 5.1.2 For example, the eddy
31、 current method is effective only when there is sufficient difference in conductivity between the coating and substrate, and the magnetic attraction and reluctance methods only when there is sufficient difference in magnetic properties between the coating and substrate. 5.2 Variations in the propert
32、ies underlying the test 5.2.1 It should not be assumed that nominally similar coatings and substrates liave identical properties. Electrolytically applied coatings, for M. 40, February, 1972 instance, may vary in density, homogeneity and metallurgical condition according to the method of production.
33、 Similarly, the properties of substrates depend on composition and heat treatment. For this reason, calibration standards are necessary, see 9. 5.2.2 Some specimens may exhibit sufficient anistropy to affect the readings given by equipment which makes simultaneous contact at two positions. In this e
34、vent, it may be advisable to standardize the direction of the pro& or to take several read- ings with the probe straddling the test area in various directions and to average the indications. 5.3 Thickness of coatings and substrates 5.3.1 There are usually limitations connected with measuring the thi
35、ckness of coatings and sub- strates, for example, tlie tendency for the un- certainty of measurement at the lower end of equip- ment scales to be approximately constant and to be a high percentage of the actual thickness to be es timated. 5.3.2 The discrimination obtainable in most instances also de
36、creases if the thickness of the coatings and substrates are respectively greater or less than certain limiting values. As an example, the beta ray back scatter method requires that the thickness of the coating should be less than ihe saturation value for the coating material and the thickness of the
37、 substrate shiiuld be greater than the saturation value for the substrate niaterial. 5.3.3 Wien such effects are due to the substrate, it may be possible to improve the accuracy of determination with out r e-cal i brat i on by teinpor ar i 1y building up the thickness with similar material placed in
38、 contact with the uncoated side. 5.3.4 In some cases, the thickness of the sub- strate may be unimportant as in tlie measurement of a nickel coating on a non-magne tic subs trate. 5.4 Geometrical effects 5.4.1 In addition to the possibility of the coating thickness itself being non-uniform, all the
39、methods under consideration tend to have limitations related to geometrical variations in the component under test. Edge effects, proximity to drilled holes, and changes of contour or section can influence the accuracy of measurement obtained by all methods unless calibration specimens representativ
40、e of the test conditions are used. 5.4.2 Variations in the surface condition of the workpiece can also reduce the accuracy of measure- ment. Precautions should be taken to repeat read- ings at nearby positions and to ensure that cali- bration specimens have a surface texture similar to the component
41、 under test. 5.4.3 Surface curvature affects measurement of coating thickness particularly when the sensing device has been designed and cdibratrci for use 3 i BSI MM40 72 I Lb24669 0099395 - 7 M i M. 40, February, 1972 on flat surfaces and when it covers a considerable area. Equipment using two-pol
42、ed probes may give different readings at the same test position accord- ing to whether the contact points are aligned longitudinally or transversely and according to the radius of curvature. Notwithstanding the above limitations and depending on the accuracy required, it may be possible to make meas
43、urements over moderately wide ranges of cylindrical curvature by calibrating equipment on the mean curvature. 5.5 Experimental procedures 5.5.1 The reliability of methods involving the application of a probe are generally adversely affected by failure to maintain presentation normal to the component
44、 under test. For example, manual magnetic attraction devices should be held and withdrawn in a direction normal to the test surface. Greater consistency can usually be obtained by using the probe with a simple mechanical guide. 5.5.2 Contact pressure may also be a critical factor and the use of spri
45、ng-loaded test coils in eddy current measurements is generally desirable. The application of a constant pressure is essential in thermo-electric tests. 5.5.3 Failure to clean the surfaces to be tested, e.g. removal of grease and loose particles, may also affect the readings. 6. SAFETY PRECAUTIONS 6.
46、1 General. In view of the range of equipment used for measuring coating thickness, it is not feasible to lay down specific instructions concerning safety and such instructions should accordingly be decided by agreement. Nevertheless, the equipment should always be operated in complete safety and in
47、accordance wiii the manufacturers instructions. Instruction leaflets wpplied by the manufacturer should contain sufficient -information for this pur- pose. All equipment involving the use of mains electricity shall conform with tiie relevant regula- tions in the Factories Act. 6.2 Ionizing radiation
48、. The instructions contained in the H.M.S.O. publication Code of practice for the protection of persons exposed to ionizing radiation and tiie Ionizing radiation (sealed sources) regulations, 1969 shall be strictly followed. 7. APPROVALS 7.1 When the use of a particular method or equip- ment for coa
49、ting thickness measurement is a contractual requirement, die inspection shall be carried gut in the manner and to the extent stipulated. 7.2 In some instances, however, the contractor may wish to replace a destructive measurement specified or implied in the contract by a non- destructive one, or to replace one non-destructive test by another. In this event, he is required to produce experimental evidence that the substitution may be justified and to propose an acceptable form of calibration specimen for control purposes. By agreement, periodic reference to a more fundame
