Inorganic tribosynthesis of solid lubricants under frictional interaction of refractory metals carbides and chalcogens A.L. Zaitsev, Yu.M. Pleskachevsky Metal-Polymer
Research Institute, Belarus Academy
of Science, 32a Kirov Street, 246 652 Gomel, Belaw
Received 4 May 1994; accepted 18 October 1994
Abstract The paper demonstrates results of experimental studies of frictional interaction between tungsten carbide and chalcogens. X-ray, metallographic and chemical analyses were used to study the kinetics of inorganic tribosynthesis of a solid lubricant, its structure, composition and frictional behaviour. The effect of external factors on inorganic tribosynthesis has been investigated. The most likely mechanisms of tribochemical reactions have been considered. A conclusion has been made that surface layers of refractory metal carbides are activated mechanically which under relatively low temperatures and the presence of chalcogens in the friction area results in transformation of carbides into chalcogenides with high frictional properties. Keywords: Tribosynthesis;
Solid lubricants; Carbides; Chalcogens
It is still a problem today how to improve wear resistance of hard alloys based on refractory metal carbides used under highly loaded sliding friction. Spalling of surface carbide grains is responsible mainly for the wear of hard alloys during friction and cutting [l-4]. The presence, as a rule, of overdeformed, very hard carbide grains within the friction area results in abrasive-mechanical wear of contacting materials. The abrasive effect of wear particles, formed after carbide grains failure, can be eliminated if these particles are removed from the friction area or converted into other compounds having high wear-resistant properties. More than 25 years ago Bowden and his colleagues investigated the possibility of forming solid lubricants on the surfaces of refractory metals carbides and silicides using the method of gas phase high-temperature synthesis. Their paper  demonstrated that sintered hard alloys were capable of self lubrication at high temperatures in the presence of chemically active reagents. Beginning from temperatures of 400 “C the authors detected stable formation and growth of polycrystal films of molybdenum and tungsten disulphides in the atmosphere of hydrogen sulphide. Under friction this resulted in a considerable improvement of the antifriction properties and wear-resistance.
0043-1648/9.5/.$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0043-1648(94)07066-O
Due to the contemporary understanding of tribological modification of surface layers of contacting materials  it is of scientific and practical interest to investigate how hard alloy carbide constituent and wear products transform into laminated solid lubricants when chalcogens are introduced into friction area by controlling load-velocity performance of frictional interaction. The aim of this paper is to study the main mechanisms of transformation of tungsten carbides into chalcogenides under sliding friction.
2. Experimental techniques Tungsten-cobalt hard alloy BK-6M (GOST 3882-SO), containing 94 mass.% of tungsten carbides and 6 mass.% of cobalt, as well as sulphur and selenium powders, GOST 127-86 and GOST 5455-84 respectively, were used for experiments. Frictional interaction between chalcogens, containing hard alloy, was investigated within a wide range of loads and sliding speeds using end-face friction apparatus. Friction of the like hard alloy cylindrical specimens, 9 mm in diameter, was observed in the contact between flat bases of both cylinders. The experiment was run for 600 s to investigate the relationship between load-speed performance and the beginning of stable formation of tungsten chalcogenides. Optimum loads
and sliding speeds were used to study kinetic mechanism of tribochemical interaction and service life of solid lubricant before its failure and setting in frictional contact area. Frictional modification consisted in the following: 5-10 mg of chalcogen powder containing 10-100 pm particles was spread over the lower stationary hard alloy specimen. After that tested specimens (movable and stationary) were interconnected through the chalcogen interlayer and brought in frictional contact under specified load and sliding speed. Frictional force and temperature (sliding thermocouple method) were fixed during experiments. Mass wear of hard alloy specimens was evaluated by weighing to an accuracy of 0.05 mg. Before weighing, specimens were treated with an alkaline concentrated solution and washed with acetone to remove remaining particles of reagents and reaction products. Hyperbolic distribution of pressure in cylindrical support was used to calculate the coefficient of friction . Experimental data were processed and frictional characteristics were averaged over time. This allowed determination of common mechanisms of tribochemical interaction between tungsten carbides and chalcogens depending on load-speed test conditions. The test was done in dried nitrogen atmosphere to study the effect of oxidizing atmosphere on chalcogenides’ formation processes. X-ray and local-chemical analyses as well as metallographic and optical microscopy techniques were used to examine the products of tribochemical interaction.
3. Experimental results Preliminary experiments discovered that definite load-speed test conditions and introduction of the likely hard-alloy chalcogens into the friction area resulted in abrupt changes of frictional characteristics along with formation of definite structures within the friction area. Four parts with different wear rate, temperature and frictional force were detected while studying kinetic behaviour of tribochemical interaction. Time dependences (Fig. 1) are characterized by areas of low- (I), high- (II), long-term low friction (III) and setting (IV). Low values of temperature, coefficient of friction and wear rate are typical for initial stage of friction interaction which is short in time and takes about 100-200 s. Increase of friction duration up to 300-400 s results in a sudden increase of temperature, coefficient of friction and wear. At maximum values there is a gradual decrease and stabilization of the frictional characteristics. The third part which is the longest in time (0.545 ks) is characterized by minor changes of frictional properties in time. During the shorter fourth stage the frictional characteristics intend for rapid increase. Optical investigations of different stages of tribochemical reaction between WC-Co and sulphur dem-
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Fig. 1. Time dependencies of weight wear (I), temperature (2) and friction coefficient (3) at hmgstenxarbide hard alloy frictional interaction with sulphur (P= 10.4 MPa, V=O.43 m s-l).
onstrated that the beginning of friction is characterized by sulphur lubrication which is initially in a solid state. Increase of frictional temperature results in sulphur melting and boundary friction (Fig. 2(a)). Higher temperature in the second region results in the increase of sulphur melt viscosity significantly raising the coefficient of friction and wear rate of the hard alloy. With the growth of frictional characteristics the lubrication interlayer becomes a grey film with a rough surface which can melt and dissolve in carbon bisulphide and benzene (Fig. 2(b)). At the moment when frictional force decreases the film becomes smoother and unable to melt and dissolve. The third region with a relatively stable coefficient of friction and low wear rate is characterized by formation of solid lubricant smooth films in the friction area which are later gradually removed from the frictional contact area (Fig. 2(c)). Increase of wear rate in the fourth region can be explained by the increase of setting and seizure due to removal of solid lubricant from friction area. X-ray and chemical analysis of frictional interaction products discovered the phase composition of lubricant interlayer at various stages of inorganic tribosynthesis (Fig. 3). Diffraction pattern corresponds to the structure of crystal sulphur at the first friction area. The widened reflexes from tungsten carbides planes (001) and (100) are also discovered here. It means that the initial stage is characterized by dry and liquid friction along with minor setting and spalling of carbide grains. The second area is represented by complex diffraction from oxidation products and decarburization of tungsten carbides. Blurred lines, hampering identification of diffractograms, prove the existence of strong distortions in the crystalline structure of lubricant interlayer due to the influence of shear and compression stress. Formation of solid lubricant (3d area) is fixed at the emergence of intensive broadened diffraction from tungsten disulphide plane (002), containing WC and W&J additives. Murakami reagent, alkaline concentrated solution and mixture of hydrofluoric and nitric acids were used for selective etching of friction surface. It was found out
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I, rel. un. (002
‘I Fig. 3. Diffractograms
formation: I, WC-Co hard WS2 inorganic tribosynthesis
alloy; 2-4, kinetics.
15 film different accordingly
8, deg stages
Fig. 4. Hard alloy surface after WS2 formation in inert atmosphere tests (original magnifications, 100~). Dark areas, WSz, light, WC-Co.
Fig. 2. Micrograph of WC-Co hard alloy surface on different stages of solid lubricant formation: (a) first region of tribosyothesis kinetics (original magnifications, 800X); (b) second region, 800 X; (c) third and fourth regions, 180 x .
that tribochemical reaction products contain not only tungsten disulphide which can dissolve in the last of the above-mentioned reagents, but also T*-phase (W& W,Co,C) and tungsten oxides (WO,-WO,). To determine the relationship between oxidation and inorganic tribosynthesis reaction tests were run in inert gaseous atmosphere. Friction in nitrogen intensifies formation of solid lubricant and yields high output of tungsten disulphide. Hard alloy friction surface (Fig. 4) does not contain any traces of pull outs, microcutting or spalling of carbide grains. Chemical analysis revealed the absence of q*-phase sub-film accumulations. Similar results were obtained during investigation of kinetics of tungsten diselenide formation. The difference
consisted in some load increase and higher heat release in 2d area of tribochemical reaction time dependence. Subsequent experiments demonstrated that inorganic tribosynthesis can take place within a definite range uf loads and sliding speeds. Extreme curves (Fig. 5) describe the relationship between hard alloy wear rate, coefficient of friction and load when chalcogens are introduced into the friction area. Experiments revealed common behaviour, that is certain wear rate increase under low loads due to poor lubricating properties of solid-phase chalcogens. Subsequent declining of wear rate with pressure rise can be explained by chalcogen melting and liquid friction during the whole test period. Further increase of specific load results in loss of chalcogen carrying ability due to increase of melt viscosity and break of boundary film. As a result, there is setting in friction contact and partial mechanical activation of chalcogen and surface layers of hard alloy. After passing the critical loads which are sufficient for complete mechanical activation of friction area and excitation of tribochemical interaction the formation of solid lubricant takes place accompanied with a decrease of friction and wear rate. Reaching the level of specific loads which causes disruption of tungsten chal-
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Experimental data reveal the following mechanism of tribochemical interaction between tungsten-c&& hard alloy and chalcogens during sliding friction in atmospheric air. Decarburization of the thin surface layer of tungsten carbides is excited during frictional loading in areas of actual contact under the influence of high local shearing and compressive stresses. The decarburization process is accompanied by formation of tungsten subcarbide and, possibly, pure tungsten. Mechanically activated chalcogen and oxygen react with W& and W forming tungsten oxides and chalcogenides in accordance with the following reactions:
W,C + Ch -
Fig. 5. Hard alloy wear rate (1,2), friction coefficient (3,4) dependencies on specific load when sulphur (1,3) and selenium (2,4) are used as reagents: (a) V=O.43 m s-‘; (b) V=O.69 m s-‘. Arrows indicate loads of solid lubricant stable formation.
W+Chw,c + 02 w+o,-
Fig. 6. Temperature (1,2) and specific load (3,4) of chalcogenides stable formation as a function of sliding velocity. Selenium (1,3), sulphur (2,4).
cogenides film predetermines the rise of wear rate and coefficient of friction. A correlation between loading dependencies (Fig. 5(a) and 5(b)) demonstrates that variation of sliding speed affects the value of specific load which provides for stable formation of solid lubricant. When sliding speed increases up to 0.4-0.5 m s-l critical load begins to decline (Fig. 6). Higher speeds reveal the effect of chalcogen’s nature: actually, the critical load does not change for sulphur while it increases for selenium. Initial conditions of tungsten chalcogenides formation are considerably affected by frictional heat release. The lowest temperature values which are required for realization of this effect are 0.9 for sulphur and 0.7 for selenium if related to melting point. Such temperatures are typical for low sliding speeds.
WCh, wo, + co, wo,
where x = O-2. The presence of tungsten oxides in lubricating interlayer during frictional interaction in the atmospheric air assumes the existence of the second mechanism of tribochemical interaction. Chalcogenides are synthesized through formation of intermediate compounds, tungsten oxides, in the following sequence: primarily the carbides are oxidized and form oxides and then oxides are reduced by chalcogen: wc+o,WO, + Ch -
WCh, + CCh,
wo, WCh, + ChO,
The absence of W,C among the reaction products of frictional interaction between WC-Co and chalcogens in inert atmosphere assumes the existence of even the third mechanism: direct reaction between tungsten carbides and chalcogens initiated by the process of friction. Influence of friction external conditions, i.e. load and sliding speed is characterized by complex dependence. Parameters of load-speed conditions which realize the tribochemical synthesis affect each other and are determined by the nature of chalcogen and its physicochemical properties. For example, usage of selenium instead of sulphur increases specific load which is required for tribosynthesis. Effect of frictional heat release is also of certain importance. Solid lubricant is not formed at temperatures which are considerably lower than chalcogen melting point. At sliding speeds as low as 0.1 m s-’ the initial reaction point may be expected as high as 400-800 “C approaching in absolute values the temperature of tungsten chalcogen synthesis from elements under conditions of static heating. Additionally, a sharp growth of specific loads which are required for me-
chanical activation of surface layers of tungsten carbides may occur at low sliding speeds. Having analyzed the influence of friction external parameters we can assume that formation of chalcogenides during friction is associated with complex action of temperature, load and sliding speed. It seems that this process depends mainly on energy which is released and absorbed during friction. On the other hand, formation of solid lubricant depends also on speed of mechanical activation of hard alloy surface layers which in its turn is affected by load and sliding speed. When speed of mechanical activation considerably exceeds parameters which define the amount of reagent in the friction area, such as chalcogen oxidation or evaporation rates, then the period of tribochemical interaction becomes shorter reducing the wear while solid lubricant synthesis runs quicker. It should be noted that solid lubricant, formed during inorganic tribosynthesis, has good wear-resistant properties. Under optimum load-speed conditions the reacting thickness of surface layer is about 0.142 pm while good antifriction and wear-resistant properties are maintained during a long period of time due to the existence of solid lubricant layer 0.246 pm thick. Linear wear rate approaches 5X 10-l’ m m-l. This value characterizes good strength properties of chalcogenides films formed under specific loads of the order of 10-20 MPa.
5. Conclusion This paper shows that although relatively inert from a chemical point of view hard alloys based on carbides of refractory metals react with chalcogens to form highly effective wear preventive solid lubricants within the
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area of friction contact. Inorganic tribosynthesis of a solid lubricant can be realized during sliding friction in the air or in an inert gaseous atmosphere. Analysis of various feasible mechanisms of tribochemical reaction demonstrates several possible directions of inorganic tribosynthesis. Direct friction-initiated interaction between tungsten carbides and chalcogen is the most important of them. Interrelation between friction external parameters fails to define the dominating influence of any of them on the realization efficiency of inorganic tribosynthesis. Anyway it is clear that reactions of the given type are impossible without mechanical activation of surface layers and reaching a certain energy level. Practical implementation of the discovered effect allows under certain friction conditions to transform hard alloy wear products into laminated lubricants improving wear resistance.
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