Astm stp 474 1970

CHARACTERIZATION AND DETERMINATION OF EROSION RESISTANCE A symposium presented at the Seventy-second Annual Meeting AMERICAN SOCIETY FOR TESTING AND MATERIALS Atlantic City, N J., 22-27 June 1969 ASTM SPECIAL TECHNICAL PUBLICATION 474 List price $28.75 ~ l j ~ AMERICAN SOCIETY FOR TESTING AND MATERIALS 1916 Race Street, Philadelphia, Pa 19103 (~) BY A M E R I C A N SOCIETY FOR T E S T I N G AND ]V[ATERIALS Library of Congress Catalog Card Number: 72-108625 ISBN 0-8031-0063-9 NOTE The Society is not responsible, as a body, for the statements and opinions advanced in this publication Printed in Baltimore, Md October 1970 1970 Foreword This publication is a collection of most of the papers presented at the Symposium on Characterization and Determination of Erosion Resistance, organized by Committee G-2 on Erosion by Cavitation or Impingement in co-operation with the U S Office of Naval Research, at the Seventysecond Annual Meeting of the American Society for Testing and Materials held at Atlantic City, N J., 22-27 June 1969 A Thiruvengadam, Hydronautics Inc., now at Catholic University of America, presided as chairman of the symposium, and F J Heyman, Westinghouse Electric Corp., presided as co-chairman of the symposium Three of the papers presented at this symposium have been published in other ASTM publications as follows: "Surface Damage from High Velocity Flow of Lithium" by L G Hays and "Dynamic Response and Adhesion Failures of Rain Erosion Resistant Coatings" by A F Corm and A Thiruvengadam, Journal of Materials, Vol 5, No 3, Sept 1970; and "ASTM Round-Robin Test with Vibratory Cavitation and Liquid Impact Facilities of 6061-T 6511 Aluminum Alloys, 316 Stainless Steel, and Commercially Pure Nickel" by C Chao, F G Hammitt, C L Kling, T M Mitchell, and D O Rogers, Materials Research & Standards, Vol 10, No 10, Oct 1970 They therefore not appear in this volume Related ASTM Publications Erosion by Cavitation or Impingement, STP 408 (1967), $20.00 Contents Introduction Cavitation and Impact Erosion Concepts, Correlations, Controversies-P H I L L I P EISENBERG Discussion Environmentally Controlled Cavitation Test (Improvements in a Cavitating Film Erosion Test) J M HOBBS AND D RACHMAN Discussion Cavitation Damage Mechanism and Its Correlation to Physical Properties of Material MASATO HmOTSU Effect of Temperature and Pressure on Cavitation Damage in S o d i u m - S G YOUNG AND J R JOHNSTON 26 29 47 48 67 Discussion 103 Vibratory Tests in Water on the Combined Action of Cavitation and Corrosion R SCHULMEISTER 109 Comparison of Erosion Resistance of Standard Steam Turbine Blade and Shield Materials on Four Test Rigs D ~ ELLIOTT, J B MARRIOTT, AND A SMITH Discussion Cavitation Erosion of a Ship Model Propeller J H J VAN DER M~ULES Discussion Some Observations on Erosion by Cavitation and Impingement G c GOULD Discussion Toward Quantitative Prediction of Liquid Impact Erosion F J HEYMANN Discussion Experimental and Analytical Investigations on Liquid Impact ErG~ion-A THIRUVENGADAM~ S L RUDY~ AND M GUNASEKARAN Discussion A Statistically Verified Model fur Correlating Volume Loss Due to Cavitation or Liquid Impingement F G HAMMITT, Y C HUANG, C L KLING, T M MITCHELL, JR.~ AND L P SOLOMON Discussion Erosion Rate-Velocity Dependence for Materials at Supersonic S p e e d s - G F SCHMITT~ JR Discussion Rain and Sand Erosion, Phenomena of Material Destruction Caused by Repeated Loads o HOFF, W HERBERT~ AND H R I E G E R Discussion Hydrodynamic Model of Correlation of Metal Removal Rates from Repetitive Drop I m p a c t - - w D POUCHOT 127 160 162 181 182 211 212 244 249 281 288 312 323 350 353 376 383 Discussion Analogy Between Erosion Damage and Pitting of Machine Component Surfaces s P KOZIREV Cavitation Damage Resistance and Adhesion of Polymeric Overlay Materials J, Z LICHTMAN 408 409 422 STP474-EB/Oct 1970 Introduction This symposium was the third on the subject of cavitation and impingement erosion to be sponsored by ASTM, the first two having been held in 1961 and 1966, respectively The advent of modern engineering systems such as supersonic aircraft and missiles, high-speed naval craft, high tip-speed steam turbines, and liquid metal space power plants has stimulated widespread interest in the phenomenon of erosion of materials by cavitation or impingement This interest is international in scope, as witnessed by similar symposia held in Great Britain in 1965 and in Germany in 1965 and 1967 The most significant contribution of these symposia has been the establishment of a common forum for scientists and engineers working on cavitation and impingement erosion and the exchange of useful knowledge As a further step we focused the attention to a specific problem and selected the theme for this symposium as characterization and determination of erosion resistance This was reflected in several papers presented in this symposium Some have made significant advances toward definition and characterization of erosion resistance in its own right and toward establishing statistical and physical correlations between this and other material properties The ASTM round-robin tests using vibratory cavitation and liquid impact facilities and the comparative erosion tests of steam turbine blade materials in Europe are examples of the recent attempts toward standardization of existing test methods Besides, new testing techniques also have been advanced Hopefully these efforts will lead toward the generalization of erosion test results so that they can be applied to the prediction of service performance and toward quantitative and qualitative understanding of the erosion process A significant number of papers have concerned themselves toward the understanding of the process of erosion itself It was highly encouraging to note the international response to this symposium and the most enlightened discussions that followed the presentations of papers These discussions are documented fully in this volume It is our belief that this symposium has contributed greatly to the scope of the ASTM-G-2 Committee on Erosion which includes "the promotion of knowledge in the area of erosion of materials by cavitation or impingement; the development, evaluation, and correlation of test methods; and the establishment of standards." Copyright*1970 by ASTM International www.astm.org CHARACTERIZATIONAND DETERMINATION OF EROSION RESISTANCE A Thiruvengadam Associate professorof Mechanical Engineering, The Catholic University of America, Washington, D.C 20017; symposium chairman F J Heymann Senior engineer, Technology Development, Large Turbine Division, Westinghouse Electric Corp., Lester, Pa 19113; symposium co-chairman Phillip Eisenberg Cavitation and Impact Erosion-Concepts, Correlations, Controversies REFERENCE: Eisenberg, Phillip, " C a v i t a t i o n a n d I m p a c t E r o s i o n - Concepts, Correlations, Controversies," Characterization and Determination of Erosin Resistance, A S T M S T P ~7~, American Society for Testing and Materials, 1970, pp 3-28 ABSTRACT: Increasingly severe requirements for materials in modern high performance applications have re-emphasized the need for fundamental understanding of erosion processes associated with cavitation and liquid impingement In recent, years, there has been disclosed a number of concepts which are proving helpful in the description of both types of erosion in a wide range of hostile environments (high temperatures, corrosive liquids, etc.) Such probings are providing a framework for development of useful methods of analysis and prediction of the response of materials under cavitation attack and liquid impact In spite of the ingenuity of these ideas and investigations, however, the nature of the complex interactions among the many parameters involved inevitably has resulted in controversies which have yet to be resolved Perhaps the most important concepts which have made possible rational correlation attempts and show promise of a unified treatment of erosion are those of energy absorption In both types of erosion, very useful results have been obtained based on static strain energy, but a more complete and critical evaluation of these ideas must await the accumulation of data on behavior of materials at high strain rates Strain rates of interest are those associated with cavitation bubble collapse and with liquid droplet velocities typical of rain impact on highspeed aircraft and droplets in wet steam or vapor turbines Whether cavitation damage descriptions can be treated independently of the manner in which the pressures of collapsing cavities are applied shock waves or internal jet formation still requires investigation Internal jet impact is analogous to droplet impingement These problems are connected intimately with the hydrodynamics of cavitation bubbles, droplet deformation, and the physical properties of the liquid environment In the latter connection, recent work in hot liquid alkali metals has added to the background information needed to achieve useful correlations The dependence of rate of erosion on exposure time and the existence of definite "zones" of erosion (incubation, accumulation, attenuation, steady state,) now seem clearly established for both cavitation and impingement attack The mechanisms which account for this behavior, however, whether hydrodynamic, mechanical, metallurgical, or combinations of these, still require clarification Very impressive correlations have been achieved for data in the so-called steady-state zone using strain energy concepts, but even here there is a question about the "steadiness" of this zone Phenomenological fatigue President, Hydronautics, Inc., Laurel, Md 20810 Copyright* 1970 by A S T M International www.astm.org 420 CHARACTERIZATION AND DETERMINATION OF EROSION RESISTANCE FIG Indium specimen damaged by hammer blows through a layer of liquid One m a y also expect a question on the role of liquid layer thickness, since it is believed that when film thickness is small the liquid may get solidified under the action of surface forces of the bodies under contact This question can be answered by saying that the minimum possible fihn thickness is attained, in the case of dynamic contact, only when a relatively long time has elapsed since the beginning of contact, t h a t is, when the lubricant thickness is still considerable The maximum impact occurs just at the initiation of contact, since it is only at that moment that the main shock wave is formed After that the lubricant is pressed out to a minimum thickness to harden One should also bear in mind data given in Ref 17 where cavitation in lubricant layers less than 6.8 • 10-2 mm in thickness was observed In Ref 18 it is stated that vibration cavitation bubbles can occur in oil films as thin as 2.4 X 10-3 ram These and other data suggest that the lubricant is capable of maintaining its liquid structure in fairly thin layers Thus, it m a y be supposed that the surface pitting of some machine parts is a consequence of direct impact of the liquid occurring during the dynamic contact of solids through a liquid layer Acknowledgment The author wishes to t h a n k Dr Thiruvengadam for his kind assistance in the publication of this paper KOZlREV ON EROSION DAMAGE AND PITTING OF MACHINE SURFACES 421 References [1] Pinegin, S V., "Kontaktnaya prochnost' v machinakh," Mashinostroeniye, 1965 [2] Goldsmith, B., "Soudareniye tverdykh tel," Mekhanika, sbornik perevodov, No 4, 1965 (translated from English) [3] Khrushchev, M M., "Laboratorniye metodyispytaniyanaiznashivaniye materialov zubchatykh koles," Mashgiz, 1966 [4] Petrusevich, A I., "Zubchatiye peredachi," in Sbornik: "Detali mashin," Kniga No 1, Mashgiz, 1953 [5] Petrusevich, A I., "Kontakniye napriazheniya, deformatsi i kontaktnaya gidrodinamicheskaya teoria smazki," IMASh, 1950 [6] Govinda Rao, N S and Thiruvengadam, A., "Prediction of Cavitation Damage," Journal of Hydrology Division, Sept 1961 [7] Thiruvengadam, A., "The Concept of Erosion Strength," Erosion by Cavitation or Impingement, A S T M STP 408, American Society for Testing and Materials, 1967 [8] Trubin, G K., "Kontaktnaya ustalost' materialov dlia zubchatykh koles," Mashgiz, 1962 [9] Davidson, T F and Ku, P M., "The Effect of Lubricants on Gear Tooth Sue'face Fatigue," Transactions, American Society of Lubrication Engineers, Vol 1, No 1, 1958 [10] Klimov, I M., "Prochnost' azotirovannykh zubchatykh peredach," in Sbornik No 1: "Problemy kachestva i prochnosti zubchatykh peredaeh," NTO, Mashprom, 1961 [11] Martin, J B and Cameron, A., "Effect of Oil on the Pitting of Rollers," Journal of Mechanical Engineering Science, Vol 3, No 2, 1961 [12] Huffaker, G E., "Prediction of Gear Pit Point," Journal, Society of Automotive Engineers, Vol 67, No 10, 1959 [13] Heymann, F J., "On the Time Dependence of the Rate of Erosion due to Impingement or Cavitation," Erosion by Cavitation or Impingement, A S T M STP 408, American Society for Testing and Materials, 1967 [14] Pavlov, Z P., "Vynoslivost' rabochikh poverkhnostei pri peremennoi nagruzke," Vestnik Machinostroyenia, No 3, 1953 [15] Thomas, G P., "The Initial Stages of Deformation in Metals Subjected to Repeated Liquid Impact," Philosophical Transactions, Royal Society of London, Series A, Vol 206, No 1110 1966 [16] Shalnev, K K., Varga, I I., and Sebestyen, G., "Investigation of the Scale Effects of Cavitation Erosion," Philosophical Transactions, Royal Society of London, Series A, Vol 206, No 1110, 1966 [17] Doyson, D., "Issledovaniye kavitatsii v maslianom sloye, nesushchem maluyu nagruzku," Mezhdunarodnaya konferensiya po smazke i iznosu maskin," 1962 (translated from English) [18] Hunt, J B., "Cavitation in Thin Films of Lubricant," Engineer, 29 Jan 1965 J Z Lichtman~ Cavitation Damage Resistance and Adhesion of Polymeric Overlay Materials* REFERENCE: Lichtman, J Z., "Cavitation Damage Resistance and Adhesion of Polymeric Overlay Materials," Characterization and Determination of Erosion Resistance, A S T M ST P 47~, American Society for Testing and Materials, 1970, pp 422-434 ABSTRACT: This study is concerned with the general problem of cavitation erosion; in particular, with coatings which can provide protection of marine and other structures against cavitation erosion and which can remain adhered when exposed to severe hydrodynamic enviromnents Cavitation erosion studies were conducted on 25 coatings, comprising both neoprene and polyurethane formulations; the Naval Applied Science Laboratory (NASL) magnetostrietion apparatus was used as the principal test device The most erosion resistant of the coatings tested were found to be polyurethane formulations Adhesion tests on the more erosion resistant coatings showed no effect of cavitation exposure on strength of adhesion Comparison of magnetostrietion data with data obtained on the NASL rotating disk apparatus showed that cavitation intensity of the magnetostriction test (under presently maintained operational parameters) exceeded the most severe intensity developed by the rotating disk apparatus KEY WORDS; cavitation, erosion, adhesion, polymeric materials, coatings, magnetostriction tests, evaluation, tests T h i s p a p e r is c o n c e r n e d w i t h t h e i n v e s t i g a t i o n of p o l y m e r i c o v e r l a y m a t e r i a l s t o d e t e r m i n e t h e i r r e s i s t a n c e to c a v i t a t i o n d a m a g e a n d to det e r m i n e effects of c a v i t a t i o n e x p o s u r e on t h e i r a d h e s i o n p r o p e r t i e s T h e s e m a t e r i a l s w o u l d be u s e d as p r o t e c t i v e o v e r l a y s or i n l a y s on m e t a l l i c or n o n m e t a l l i c s t r u c t u r e s , such as ships' p r o p e l l e r s , h y d r o f o i l sections, a n d t u r b i n e b l a d e s , which a r e o p e r a t i n g in e a v i t a t i n g e n v i r o n m e n t s P r e v i o u s s t u d i e s of c a v i t a t i o n d a m a g e r e s i s t a n c e a n d a d h e s i o n of p o l y meric m a t e r i a l s a t N a v a l A p p l i e d Science L a b o r a t o r y ( N A S L ) h a v e b e e n c o n d u c t e d u n d e r h i g h - s p e e d h y d r o d y n a m i c c o n d i t i o n s using a r o t a t i n g disk a p p a r a t u s [1-4] T h e p r e s e n t i n v e s t i g a t i o n s were c o n d u c t e d using a * The opinions or assertions contained in this paper are the private ones of the author and are not to be construed as official or reflecting the views of the Naval Service Materials engineer, Naval Applied Science Laboratory, Brooklyn, N Y 11251 The italic numbers in brackets refer to the list of references appended to this paper 422 Copyright*1970 by ASTM International www.astm.org LICHTMAN ON POLYMERICOVERLAY MATERIALS high-frequency vibratory facility (magnetostriction apparatus) [3, 5, 6] which creates a cavitating environment under relatively static fluid flow conditions Each of these units can be operated to produce a range of cavitation intensities, t h a t is, by control of disk speed and other cavitation factors in the rotating disk machine, and by frequency and amplitude in the magnetostriction device Comparative studies were made between the two devices under specific operating conditions [5, 6] The present investigation also contributes to this comparative study since several of the materials were tested previously by the rotating disk method Earlier studies [1, 5] indicated t h a t one of the vulnerable areas in the performance of a protective overlay in a cavitating environment was failure of the adhesive bond between overlay and substrate The present investigation of adhesive bond strength, after cavitation exposure, for those overlays which demonstrated a high degree of erosion resistance will contribute to an understanding of the effects of cavitation exposure on adhesion of polymeric overlays Objectives The objectives of the investigations described herein were: To determine the effectiveness of a number of polymeric overlay systems in resisting cavitation erosion To determine the correlation of cavitation intensities in magnetostriction and rotating disk cavitation test facilities To determine the effects of exposure to high intensity cavitation environments on the adhesion of these overlay systems Overlay Materials The polymeric overlay materials used in this investigation are listed in Table TABLE Polymeric overlay materials Polymer Type Neoprene (NS) Neoprene (NL) Polyurethane-ester (PES) Polyurethane-ether (PET) Polyurethane~ (PNI) Physical Form Formulations Tested cured sheet, adhesive bonded liquid, cured-in-place cured sheet, adhesive bonded cured sheet, adhesive bonded cured sheet, adhesive bonded 2 10 Total number tested 25 No further chemical identification was provided by suppliers of the materials 424 CHARACTERIZATION AND DETERMINATION OF EROSION RESISTANCE T A B L E Mechanical properties of overlays Polymer Type Overlay Thick- Tensile UltiDuromDesigness, Strength, mate eter T e a t Strength, nation mils psi Elonga~ H a r d lb/in b c tion, % hess, d Neoprene, sheet NS-1 NS-2 65 59 2250 1975 350 500 A67 A78 44 Neoprene, liquid NL-1 NL-2 24 25 1890 2065 860 1180 A80 A60 160 80 PES-1 63 65 70 56 6500 5500 3000 500 300 150 68 s ;dd 1)25 A73 A78 A80 A68 i7; 58 5000 550 A78 PET-1 82 65 64 119 90 5500 6000 4000 7000 4500 , D35 D42 A80 D67 D46 Polyurethane~ s h e e t PNI-1 10 78 76 72 82 78 76 59 90 61 52 6300 6300 4590 675 700 700 4"500 6200 45() 530 Potyurethune-ester sheet Polyurethane-ether sheet 2()0 400 D32 D30 A81 A73 A73 ])37 D34 A80 I)38 A83 750 400 575 580 580 250 140 390 39O i20 Polymer type not identified by suppliers of materials in this group b A S T M 624-54, ]Die C A S T M D 470-64 Split tear Specimen in by i n , ~ in split9 d A S T M Method D 2240-647 Some specimens were "rigid" requiring D-durometer measurements Hardnesses greater t h a n A-85 were determined b y D-durometer (A 85 approximates D 15) M e c h a n i c a l p r o p e r t y d a t a of t h e o v e r l a y m a t e r i a l s a r e g i v e n i n T a b l e The hardness and split tear strength data were obtained from NASL tests; the tensile strength, percent ultimate elongation, and Die C tear data were provided by the material suppliers Overlays PNI through were formulated from the same urethane p r e p o l y m e r T h e s e five f o r m u l a t i o n s d i f f e r e d i n r e s p e c t t o a m o u n t a n d t y p e of c u r a t i v e , p l a s t i c i z e r , a n d c a t a l y s t LICHTMAN ON POLYMERIC OVERLAY MATERIALS 425 Method Cavitation Erosion Apparatus Cavitation erosion resistance of the test overlays was determined by using the NASL magnetostriction apparatus This apparatus has been described in Refs 3, 5, and 6, and consists essentially of a transducer stack, an exponential horn attached thereto, and electronic equipment for driving and monitoring the transducer The short, cylindrical test plugs, 5~ in diameter, are attached to the free end of the horn During test, the free end of the horn is immersed in distilled water and driven so that the face of the plug vibrates perpendicularly at a fixed frequency and amplitude The test specimen exposed to cavitation is the face of the plug or a test specimen attached to it In this study, the test specimens were adhered to the face of the plug Procedure The coatings were applied to the grit-blasted flat surfaces of the ~-in.-diameter mild steel plugs The liquid neoprene coatings were applied to the faces of the test plugs by brush The cured sheet materials were attached to the faces of the test plugs by use of a two-part, roomtemperature curing paste-type epoxy adhesive Wafers, a/~ in in diameter, of the sheet materials were buffed with a No 80 fine grit power disk on FIG Neoprene specimens after magnetostriction exposure 426 CHARACTERIZATION AND DETERMINATION OF EROSION RESISTANCE the surface to be bonded, and bonded with 3M 1838 B/A epoxy adhesive After cure of the adhesive, the coating margins were buffed to the 5/~ in diameter of the steel plugs A specimen (plug and bonded overlay) was screwed into the lower end of the 13.5 kHz exponential horn using a silicone grease lubricant and tightened by wrench After immersion of the specimen in distilled water to a ~ in depth, the vibration frequency was adjusted to resonance (approximately 13.5 kHz), and power raised to obtain a vibration of approximately mils double amplitude The specimens were FIG Polyurethane-ester specimens after magnetostriction exposure LICHTMAN ON POLYMERIC OVERLAY MATERIALS 427 FIG Polyurethane-ether specimens after magnetostriction exposure ~xamined at intervals during exposure, and a test was terminated when iamage to the overlay (perforation, blistering, erosion, eruptions or cracks) was observed If no damage was observed, exposure was stopped after 10 h T h e temperature of the distilled water was maintained at approximately 75 F throughout exposures One specimen of each of the sheet overlay materials and two specimens of each of the two liquid neoprene coatings were tested 428 CHARACTERIZATION AND DETERMINATION OF EROSION RESISTANCE FIG Polyurethane specimens after magnetostriction exposure Adhesion Properties Peel tests (90 deg, ASTM Tests for Adhesion to Vulcanized Rubber to Metal, Method B (D429-64), were conducted on the six specimens which showed no damage of the overlay in the 10-h magnetostriction test The peel strips was 1/~ in wide, cut through the middle of the 5~ in diameter overlay bonded to the plug The rate of peeling was in./min The peel load was plotted autographically to observe load variations during peeling Specimens which were damaged during cavitation exposure also were LICHTMAN ON POLYMERIC OVERLAY MATERIALS 429 T A B L E Results of magnetostriction tests Overlay Designation Exposure Time, h :rain Condition of Overlay After Exposure NS-1 10:05 erosion at center NS-2 2: 00 erosion at center NL-1 2:35 erosion at center NL-2 : 53 erosion at center PES-1 PET-1 PNI-1 10 5: 55 2:00 10:00 2:00 2:05 2:10 10:30 10:10 2:00 2:25 10:00 10:30 10:00 2:00 2:00 0:01 2:00 10:50 2:00 7:40 10:00 erosion at center cracks at center no damage eruption, cracks blisters, eruptions erosion at center no damage no damage cracks at center extensive erosion no damage slight scuffing at center, mm dia no damage erosion at center erosion at center, cracks blisters erosion at center no damage erosion at center, cracks erosion, eruption at center erosion at center e x a m i n e d v i s u a l l y a f t e r c u t t i n g a w a y t h e o v e r l a y t o d e t e r m i n e effects o n the adhesive interface Results Cavitation Damage Resistance T h e d a m a g e r e s i s t a n c e of t h e p o l y m e r i c m a t e r i a l s as i n d i c a t e d b y t h e i r c o n d i t i o n a f t e r m a g n e t o s t r i c t i o n e x p o s u r e is s u m m a r i z e d in T a b l e s a n d T h e a p p e a r a n c e of t h e c o a t i n g s a f t e r e x p o s u r e is s h o w n in Figs t h r o u g h N i n e t e e n m a t e r i a l s s h o w e d d a m a g e w i t h i n 10 h, o n e s h o w i n g d a m a g e i n as l i t t l e as rain Six m a t e r i a l s s h o w e d n o d a m a g e in 10 h, i n d i c a t i v e of high erosion resistance In this period, even highly erosion-resistant metals, s u c h as S t e l l i t e 6B, h a v e s h o w n m e a s u r e a b l e e r o s i o n [5, 6] A c o m p a r i s o n of t h e d a m a g e s u s t a i n e d b y s e v e r a l of t h e p o l y m e r i c m a t e r i a l s in m a g n e t o s t r i c t i o n a n d r o t a t i n g d i s k t e s t s is s h o w n in T a b l e Neoprene (NS) cured sheet, adhesive bonded Neoprene, liquid (NL) Polyurethane Ester (PES) E t h e r (PET) Type not identified (PNI) Totals Polymer Type 1 2 1 1 1 1 1 Erosion Blisters, Extensive Blisters Erosion, Erosion, Cracks at Slight No at Center Eruption Erosion Eruption Cracks Center Scuffing Damage N u m b e r of Coatings T A B L E Erosion resistance of polymeric materials as shown by condition of coating after magnetostriction exposure z 0 z z o O ~ z~y z N -r LICHTMAN ON POLYMERIC OVERLAY MATERIALS 431 T A B L E Comparison of magnetostriction and rotating disk tests Overlay Designation Magnetostriction Tests T i m e , h: rain NS-1 NS-2 NL-1 NL-2 PES-6 10:05 : 00 2:35 1:53 2:10 Condition erosion erosion erosion erosion erosion at at at at at center center center center center Rotating Disk Tests, 150 f t / s , 15 p s i g Time, h Condition 14 24 15 no d a m a g e slight scuffing s l i g h t scuffing no damage no d a m a g e F I G Polyurethane specimens after magnetostriction exposure 432 CHARACTERIZATION AND DETERMINATION OF EROSION RESISTANCE TABLE Results of adhesion tests Overlay Designation PES-3 PET-1 PET-2 PET-5 PNI-2 PNI-7 Adhesive Strength, lb/in, width Mode of Separation 8.8 34.0 26.8 22.8 25.6 ~ 53.6 coating-adhesive coating-adhesive coating-adhesive coating-adhesive coating-adhesive coating tear, no separation Adhesion Properties The adhesive strengths and mode of separation of the six overlays, which showed no erosion after a 10-h exposure period, are given in Table The load on the 90 deg tab of the overlay, when plotted autographically during the adhesive test, showed no change in load in the center areas of the specimens where the cavitation intensity has been a maximum The coatings which did show erosion damage, as indicated in Table 3, were cut from the adhesive stratum of the specimens using a razor blade None of the specimens showed local failure at the adhesive interface despite the cavitation damage to the outer regions of the coating Discussion A comparison of properties, Table 2, of the six materials which showed no erosion damage in a 10-h exposure period, with those of the other materials, does not indicate a clear correlation between erosion resistance and tensile strength Materials which had higher strength values than those of the most erosion resistant of Table 3, evidenced lower erosion resistance Only the ether polyurethane P E T - , which had the highest tensile strength and hardness, showed severe erosion; this was probably due to its high hardness Screening of materials for optimum resistance to cavitation erosion, therefore, appears to be more successful by direct evaluation of this property itself rather than the indirect (and presently inconclusive) evaluation on the basis of other (essentially quasistatic) mechanical properties Preliminary screening on the basis of polymer type and mechanical properties may be feasible, however, in view of the higher erosion resistance of the high-strength ether type polyurethanes within a hardness range of approximately D 30 to 46 Conclusions Cavitation Damage Resistance: The overlay materials show a broad range in resistance to damage, several showing no damage in 10 h, and others showing damage in much shorter exposure intervals LICHTMAN ON POLYMERIC OVERLAY MATERIALS 3 The nature of damage varied considerably, in some instances occurring as internal failures of the material rather than as surface erosion or fatigue cracking On the basis of the limited number of formulations tested, the highstrength ether-type polyurethanes in a hardness range of D 30 to 46 appear to offer a higher degree of erosion resistance than the other polymer types Precise correlations between erosion resistance and specific mechanical properties of polymeric materials are not indicated Differences in formulation of overlay materials, using the same urethane prepolymer, have significant effects on the cavitation damage resistance, as well as other mechanical properties of the overlay materials, as shown by overlays P N I - through In view of the lack of such correlations, the erosion resistance of a polymeric material may be identified directly by a suitable erosion test method, such as the magnetostriction method described herein The magnetostriction apparatus and procedures described were effective in identifying the resistance of polymeric materials to damage when exposed to a cavitating liquid The erosion intensity of the magnetostriction method is higher than t h a t of the rotating disk method under the specific test conditions used Adhesion Properties Cavitation exposure had no significant effect on the adhesive interfaces nor on the adhesive strength of the polymeric materials tested Changes in the adhesive interface did not occur during the exposure periods and therefore did not contribute to the cavitation damage observed Acknowledgment This investigation was conducted under the sponsorship of the Naval Ship Systems Command and the Naval Ship Engineering Center, and the cognizance of J L Schuler, E A Bukzin, and D P r a t t of these activities The author wishes to acknowledge his gratitude to A Rufolo and D H Kallas for their valuable discussions, criticisms, and reviews of this paper References [1] Kallas, D H and Lichtman, J Z., "Cavitation Erosion," Environmental Effects on Polymeric Materials, Vol 1, Interscience, New York, 1968, Chapter [2] Lichtman, J Z., "Cavitation Erosion Performance and Related Properties of Cured Sheet Elastomeric Coating Systems," Journal of Materials, Vol 2, No 3, 1967, pp 638-660 [3] Lichtman, J Z and Kallas, D H., "Erosion Resistance of Coatings," Materials Protection, Vol 6, No 4, April 1967, pp 40-45 434 CHARACTERIZATION AND DETERMINATION OF EROSION RESISTANCE [h] Kallas, D H., Lichtman, J Z., and Chatten, C K., "Cavitation Erosion Resistant Coating," Proceedings, Seventh Joint Army-Navy-Air Force Conference on Elastomer Research and Development, ONR Publication ONR-13, Oct 1962, pp 422-442 [5] Lichtman, J Z., Kallas, D H., and Rufolo, A ,"Cavitation Erosion Facilities and Developments at the U S Naval Applied Science Laboratory," Norwegian Ship Model Experiment Tank, Publication No 99, Vol 1, Dec 1967, pp 273-320 [6] Lichtman, J Z., Kallas, D H., and Sorkin, G., "Cavitation Erosion of Materials," Second International Congress on Marine Corrosion and Fouling, Athens, Greece, Sept 1968