F R I C T I O N AND WEAR IN A G G R E S S I V E M E D I A M, D. Sinyavskaya Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 2, No. 1, pp. 78-83, 1966 New tectmological developments, particularly in the chemical engineering, instrument making, rocket and turbine construction, and nuclear power industries, demand means of ensuring satisfactory functioning of friction pairs in such media as acid and alkaline solutions, concentrated acids, distilled and super-high purity water, sea water, molten salts, liquid metals, etc. ; often these friction pairs operate at elevated temperatures, increased pressures and high sliding speeds. In view of this, research workers in the field of friction and wear are faced with a number of important problems, which include selection of materials of sufficiently high wear- and corrosion-resistance to be suitable for the fabrication of heavy duty bearings, studies of complex interrelated physical and chemical phenomena taking place on the rubbing surfaces, and the development of new materials with specific properties such as sintered cermets. Friction pairs are subjected to simultaneous mechanical and corrosion-induced wear. Since these two kinds of wear are interrelated, the problem of wear resistance of friction pairs cannot be solved by studying these phenomena separately; this is particularly so because the rate of wear of both metallic and nonmetallic materials depends on the strength of a ~third body," such as protective (most often oxide) films or products of wear. Consequently, apart from the mechanical characteristics (extent of wear, friction coefficients), it is necessary to know the electrochemical parameters (e. g., the electrode potential) which determine the corrosion behavior of a friction pair in service. The effect of electrochemical processes on the mechanical properties of materials and on their fatigue characteristics have been quite extensively studied by P. A. Rebinder, G. V. Karpenko, and others. Much less attention hasbeen directed to studies of the effect of corrosive media on friction and wear. Investigation of Friction-Induced Wear of Materials in Aqueous Solutions It was shown in [1] that studies of wear of metals in corrosive liquids require the application of special techniques and equipment, because the effect of an aggresive medium may substantially vary depending on the quantity of the medium and the method of feeding it to the rubbing surfaces, on the temperature of the medium, and on other factors. The experiments described in [1] were carried out on a modernized machine Kh2-M, the extent of wear being measured in terms of the dimensions of the lune cut in the specimen surface by a rotating disc. The friction pair was immersed in an aggresive medium which could be heated if necessary. Various materials including elcctrodeposited coatings were studied by this method which, although useful for screening or comparative tests, does not yield results that could be used to predict the performance of friction pairs under actual service conditions. Studies of wear of materials of sliding bearings operating in acid and alkaline media have been carried out at the

NIIKhlMMASh

since 1958 [2, 3].

In some of the experimental work carried out by I. V. Vastl'ev, the method described in [1] was used. Other tests were carried out on a type MI-1 machine, whose main shortcoming lies in the method of feeding the lubricant: during each revolution of the roller each point of its rubbing surface passes altemateiy through the electrolyte and through air, which is bound to promote the formation of surface oxide films. This shortcoming was eliminated in subsequent experiments which were carried out on a type MT-2 machine (constructed at the NIIKhlMMASh), and in which the operating conditions of a bushing shaft unit could be simulated. The tests were carried out on steel 20 and 45 specimens in distilled water, and in 1-80% strong alkaline solutions; the general conclusion was that both the friction coefficient and the rate of wear of the steels studied initially increase with increasing NaOH concentration, reach a maximum at about 1-2% NaOH, and then sharply decrease. Vasil'ev studied also the condition and surface finish of the rubbing surface layers and their stress state, as well as the corrosion-resistance of the friction pair components; he remarked on the fact that surface roughness promotes intensive wear.

Work reported in [2], where a method of measuring electrochemical potentials during friction is described for the firsttime, is of particular interest. Variation in electrochemical potentials is associated with changes in physical and chemical properties; consequently, if the coefficient of friction, extent of wear, and electrochemical potential are known, it is possible to draw inferences regarding the character of phenomena taking place on the rubbing surfaces.

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A method of measuring the surface potential during corrosion-fatigue tests on steel was developed by Ryabchenkov [4], while Tomashev [5] used a special method of cleaning m e t a l surfaces in an electrolyte in his studies of passivation

phenomena. With the aid of these techniques it was established that surface films formed on friction pair components shift the metal potential toward the positive values; when the film is removed, the potential becomes more negative. This means that the electrode potential is closely related to the wear of surface films and depends on the sliding speed, load and electrolyte temperature. It was for this reason that methods of measuring the surface potential of stationary and rotating parts (with and without surface films) was developed and used in tests on various materials in various ambient media. A n equally interesting investigation, concerned with the selection of materials for sliding bearings operating in 1% N a O H solutions, was carried out on sintered materials impregnated with Teflon-4. The antifrictionproperties of sintered iron, bronze, stainlesssteel, and chromized iron powders under dry friction conditions are improved by impregnating them with teflon; this treatment is less effective in the case of materials operating in I% N a O H solutions owing to the cooling action of the liquid medium which reduces the self-lubricating effect of teflon. Investigation of the Wear of Materials in Nitric Acid One of the combinations characterized by high wear-resistance in hot concentrated nitric acid is t e f l o n - i m p r e g nated sintered chromized low-carbon steel rubbing against pure chromium: In a 23 hr test a friction coefficient of 0. 003 was recorded, the degree of wear of the chromium part amounting to 0 . 4 micron. Table I Shaft m a t e r i a l Stainless steel Khl5N9Yu

Sintered bearing m a t e r i a l Bronze (32% porosity) impregnated with teflon (6~ Stainless steel impregnated with teflon T e f l o n - i m p r e g n a t e d low-cathon steel

Coefficient of friction 0.04 0.05 0.06

The results of laboratory tests showed that the following can be recommended as the bearing m a t e r i a l s for steel 1KhlSNgT parts operating in nitric acid: t i t a n i u m - b e a r i n g stainless steel impregnated with 12-15% teflon, pure t i t a n ium, or titaninm-stainless steel mixtures. investigation of Wear in Sea Water In contrast to data on the corrosion-resistance of various materials in sea water, l i t t l e information can be found in reference books on the effect of this m e d i u m on their wear-resistance properties. Some materials recommended for service in sea water on the basis of tests carried out at a sliding speed of 1 m / s e e and under a load of 5 k g / c m 2 are listed in T a b l e 1. Other materials recommended for this purpose include phosphor or aluminum bronzes [6]. By means of tests oil a type MI-1 machine with recirculated sea water [6], it was established that stainless steel gives a better performance in friction in this m e d i u m than in air; the stainless steel-bronze combination in sea water at normal temperatures also gives a satisfactory performance. When sea water is used in a closed system, corrosion inhibirors can be employed; for instance, the rate of corrosion of low-carbon steel can be reduced by the addition of 1-4o]o sodium nitrite [6]. Wear in Pure Water Materials used as heat transfer m e d i a in nuclear reactors include water, liquid metals and gases. Units and a g gregates designed to pump these m e d i a require antifriction materials which are resistant to the action of these substances; water used as the heat transfer m e d i u m must be of a high degree of purity. Research workers are therefore faced with the problem of developing materials which are stable in high purity water at temperatures of up to 400"C and which have good antifriction characteristics. The problem of corrosion of metals in pure water is dealt with in [7, 8], but there is only one source [9] In which both corrosion and wear are treated at the same t i m e . This book is a summary of data published mostly in the American journals before 1957.

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All these investigations were concerned mainly with static corrosion tests and both laboratory and operational wear tests. Their results showed that materials which have a good corrosion resistance cause i m m e d i a t e seizure or, at best, can function for very short periods only. It was found that soft aud plastic bearing materials operating in high purity water corrode very rapidly and are not suitable for use in the first reactor circuit. Subsequently, the wear-resistance properties of hard (HRC = 30-60) materials were therefore studied. The search for materials of friction pair components was carried out with two objects in mind: determination of the optimum combinations of hard corrosion-resistant materials, and development of materials comprising a hard matrix with soft material inclusions ( e . g . , impregnated sintered materials). Comparative wear tests were carried out in autoclaves on specimens simulating the piston cylinder and bushing shaft assemblies operating in water at temperatures of up to 260"C, under a load of 6.7 k g / c m z. The rate of wear was determined from the weight losses and dimensional changes. The best combinations of materials for the bushing shaft assemblies, selected on the basis of the results of a large number of tests, are given in Table 2. (A friction pair with a wear coefficient of up to 22 is regarded as having satisfactory wear-resistant properties. ) A series of more extensive tests was subsequently carried out on some of the more satisfactory combinations. Table 2

Shaft m a t e r i a l

Bushing m a t e r i a l

Coefficient of wear, m / / k g (for 103 loading c y cles).

Steel AISI 440C (~1.2~ 16-18%Cr, 0.45%Mo).

Steel AISI 440C

6.6

Chromized honed steel tT-4PH (< 0.07%C, < l%M, 16-18%Cr,

Lead

8.8

Silicon bronze

8.8

3-4% Cu, < l%Si, 0.04% P, 3-5%Ni, Nb and Ta)

Stellite No. 3 (33% Cr

13

3% Ni, 2.7% C, I3% W, Co) w

R

Steel AISI 440C

Steel 2Kh13 3hromium-molybdenum steel Chromized honed steel 17-4 PN

6.6 11 22

Temperature variations have a substantial effect on the m a t e r i a l wear; it is stated, as a general conclusion, that the extent of wear at 260~ is 10 t i m e s greater than at 93~ tn selecting materials for friction pair components it is necessary to determine their optimum surface finish and the optimum clearances between parts m a d e of materials with different thermal expansion coefficients. An effective method of increasing the wear resistance of friction pairs is surface alloying and the use of various surface coatings. Best results were obtained with nitrided titanium and chromium, followed by c h r o m i u m - c o a t e d materials; it was found that the harder the basts m a t e r i a l the higher is the load which a chromium coating can sustain. Coatings of c o b a l t - b a s e alloys (stellites) are widely used; since, however, the deposition of such coatings is a complex process, it is more convenient to use chromium plating. Combinations comprising a m e t a l shaft rotating in contact with a sintered m e t a l or c e r m e t m a t e r i a l were also i n vestigated. Satisfactory results in tests at 260~ were obtained on g r a p h i t e - c a r b i d e compositions, ceramics and plastics. Materials consisting of chromium and aluminum were found to have a very high resistance to wear. T e f l o n - c o a t e d graphiter-4 and steel 17-4RN are most widely used in applications in which the service temperature does not exceed 93~

60

Summary 1. New technological developments necessitate extension of the theory of friction and wear to cover corrosionmechanical wear. This will involve fundamental studies of phenomena taking place on rubbing surfaces necessitating the use of metallographic, electron microscope, X-ray diffraction and electrochemical analyses. 2. In studies of corrosion-mechanical wear it is essential to carry out field trials, since this kind of wear is substantially affected by the hydrodynamic action of the lubricant, method of feeding the lubricant, temperature of the ambient medium, sliding speed, and pressure; the combined effect of all these factors is difficult to reproduce in laboratory tests. 3. With a few exceptiom, all published papers give information on the selection of materials for service under specified conditions without reference to the fundamental aspects of phenomena taking place on rubbing surfaces. 4. According to published data, friction pairs comprising sintered components possess satisfactory wear-resistance in corrosive media. REFERENCES 1. M. M. Khrushchev and M. A. Babichev, collection: Friction and Wear of Machines [in Russian], Izd. AN SSSP~ no. I0, 1955. 2. I. V. Vasil'ev, Transactions of NIIKhlMMASh: Corrosion and Wear of Metals [in Russian], no. 27, 37. 3. I. V. Vasil'ev, collection: Friction and Wear of Machines [in Russian], Izd. AN SSSR, no. 11, 1961. 4. A. V. Ryabchenkov, Corrosion-Fatigue Strength of Steel [in Russian], Moscow, 1953. 5. I. D. Tomashev, G. P. Chemova, et al., Z!, no. 3, 1958. 6. Transactions of the Central Naval Scientific Research Institute: Problems of Metallography in Ship Repair [in Russian], Leningrad, 1956. 7. Corrosion in High-Purity Water [Russian translation], IL, 1958. 8. Corrosion of Reactor Materials [in Russian], Moscow, 1960. 9. Corrosion and Wear in Water-Cooled Reactors [in Russian], Sudpromgiz, Leningrad, 1959. 15 October 1964

Institute of Materials Science AS UkrSSR, K~ev

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