STUDY OF SUPERFICIAL LAYER AT WOOD BASE COMPOSITES USED IN SLIDING TRIBOSYSTEMS

Transcription

1 NATIONAL 107 STUDY OF SUPERFICIAL LAYER AT WOOD BASE COMPOSITES USED IN SLIDING TRIBOSYSTEMS Mioara Hapenciuc, Mioara Bucşă, Constantin Gheorghieş, Aurel Hapenciuc University Dunărea de Jos Galaţi, Romania ABSTRACT The paper presents experimental researches done on a sliding tribomodel, roller-shoe type. The material couple is steel high-densified wood that is a wood based composite material. The tribomodel works with water lubrication and with oil-in-water emulsion. The tribological behaviour of the material couple is analysed from the point of view of superficial layer parameters. These parameters are chemical characteristics, mechanical and physical characteristics, surface roughness, tribological characteristics (wear, friction coefficient). The tribolayer will be physically characterised by the real contact area, the evolution of the surface roughness, the temperature regime, wear, and chemically by specific processes (as absorption and adsorption). KEYWORDS: tribomodel, tribolayer, absorption, adsorption, wear. 1. SUPERFICIAL LAYER OF SLIDING TRIBOMODELS The structure of a tribosystem has been added with important elements for its durability, meaning the superficial layers (fig. 1), considered as material volumes that participate and are directly influenced by friction processes and wear in the contact area. In 1970, Godet (INSA Lyon) introduced the concept of the third element : any contact between two bodies in relative movement involves the third element, the lubricant, with elastic-plastic properties, which rheological properties are less known [9]. The concept of tribolayer appears in the same time in Crudu s paper [7] and in tribology magazines in 1994 [1]. Fig. 1 [7] The structure of a sliding tribosystem 1 and 2-triboelements, 3-lubricant, 4 -environment, 5 tribolayers. The experimental studies presented in this paper consider that any tribosystem works in two phases: the initial phase and the actual working phase. In the first phase the processes that appear between the two surfaces in relative motion have high gradients (Fig. 2). The research is made on tribomodel but some of the sliding bearing parameters in work are the same as for tribosystem. 2. THE SUPERFICIAL LAYER OF NON-METALLIC COMPOSITES For ceramics and plastics in dry contact, Caubet model [6] (Fig. 3) is generally available, but the processes in sliding under load are specific. In the case of composites models the problem is more complex. That is why the direct observation of the superficial layer and creating models based on this observation, are preferred. For machine elements made of plastics or their composites the superficial layer depends on a package of factors, similar to that of metallic alloys but with particular aspects. Considering the evolution of a triboelement superficial layer, this must be characterised in three different stages: I the initial superficial layer, after the final polishing of the element; II the tribolayer or superficial layer made while the element is working; III the superficial layer after the movement stops. The direct study of the tribolayer is difficult to realise because any measurement device introduced into the superficial layer or in the nearby, modifies the local processes. Most of the studies are based on the observation of the superficial layer before and after functioning. The goal is to find evolution laws of some parameters or processes that can be used later, in design [1, 3, 5, 14].

2 108 NATIONAL S SO - parameters of the initial superficial layer: roughness, chemical composition, structure, purity, stress, physical, chemical, mechanical characteristics, C TO - initial constructive characteristics: shape, dimensions, clearance, thickness of added layers, S S r - tribolayer parameters after the running-in stage C T r - constructive characteristics after the running-in stage S T e - tribolayer parameters in working C T e - constructive characteristics in working S T e ~ S T cr and/or C T e ~ C T cr means that the tribosystem is inefficient, with big probability of failure 1, 2, 3, - energetic parameters (efficiency, temperature, heat), mass loss (wear, wear intensity, ) Fig. 2 Characteristic parameters of the sliding tribosystem [7]. Superficial layer - chemical characteristics - physical and mechanical characteristics composition, structure, crystallisation degree, impurities, components of specific reactions with materials in contact - roughness - initial and final stresses - limits at different static and fatigue loading - thermal characteristics - rheological characteristics changing phase temperature, melting, solidifying, etc., specific heat, thermal conductibility, radiation coefficient, dilatation coefficient - surface - roughness (form, size, distribution) topography - errors from the ideal shape, initial errors, errors appeared in working - tribological - friction coefficient characteristics - wear Fig. 4 Characteristic parameters of the superficial layer [7] Fig. 3 Caubet s model [6]. 1 metallic base layer; 2 deformed layer; 3 layer of oxides; 4 adsorption layer; 5 impurities. Figure 4 presents a characterisation of the superficial layer for plastics and their composites [7, 8, 15]. Addition materials are complicating the scheme: The superficial layer of a composite has different values for its parameters, depending on concentration, orientation, and component position, comparing to base material. The tribolayer can be characterised by: real contact area, surface topography evolution [15], thermal regime, specific processes (deformation, braking of crystalline aggregates, softening, wear, and cracks). Experimental researches of superficial layer made on tribomodel are very important, because the tribomodel behaviour is similar to that of the real tribosystem and the microscopic processes are very much alike. That is the case only if command parameters of the tribosystem are kept in a certain range: construction parameters (dimensions l/d, geometry) and working parameters (cinematic as speed, energetic as load, lubricant, environment). Important information on the processes within the superficial layer will be obtained by comparing initial parameters with the final ones. Other aspects giving indirect information, may be: lubricant analysis, shape and size of wear debris, friction coefficient, noise and vibration, wear.

3 NATIONAL CONSIDERATIONS ON OF COMPOSITES WITH WOOD AND PHENOL- FORMALDEHYDE RESINS Composites with wood and phenolformaldehyde resins are thermosetting materials, part of phenoplasts category. They can be different after their structure and manufacturing technology: pressing phenoplasts, stratification phenoplasts. The tribological behaviour can be determined by analysing the friction coefficient, wear resistance and running in capacity. These parameters are influenced by certain physical-mechanical properties: liquid absorption capacity, elasticity and plasticity, thermal stability and conductance, certain phenomena that appear in work due to rheological behaviour. For the multilayer composites the friction coefficient depends on the wood fibbers position comparing to the friction surface. The most important characteristic of the wooden filling is that absorbs liquids or polymers and the initial dimensions become bigger. Raising the resin concentration and the pressing force will reduce this inconvenient [2]. Wooden composites based on phenol-formaldehyde resins have a higher elasticity and plasticity, under load, than metals [2, 11]. Elasticity means that the initial dimensions are the same after the loading stops; elastic recover lasts in the first 8 hours. This involves a running in period after a stop longer than 3 hours. Plasticity is shown by wooden composites with capacity of well adjusting on the shaft, thus on the actual surface of micro-contacts there are no load concentration. Deformation under load due to elasticity and plasticity depends on the bearing thickness. The thermal conductibility is small. That is why the friction heat is difficult to be evacuated, and this influences the linear dimensions. The phenomenon is more obvious at polymers due to high thermal dilatation coefficient. This is important for bearing clearance calculation, in order to avoid gripping [11]. Experimental research made on wood-based composites with phenol-formaldehyde allowed deciding that they belong to viscous-elastic materials category [2, 11]. These materials, under loading suffer deformations in one or many more directions. Total deformation is made of instantaneous elastic deformation, viscous-elastic deformation and plastic deformation. Short testing proved that instantaneous elastic deformation is much higher as compared to that of the other two [11]. Regarding fibber position towards the friction surface, it has been experimentally proved that a good behaviour is in the case when fibbers are perpendicular to the friction surface. 4. CHANGING IN THE SUPERFICIAL LAYER OF WOOD BASED COMPOSITES WITH PHENOL- FORMALDEHYDE RESINS Due to the structure of composites with wood based composites with phenol-formaldehyde resins the results presented in section 2 of this paper can be considered for analysing the superficial layer of these materials. Parameters presented in figure 4 are the basic elements of this analysis. In stable sliding on smooth metallic surfaces there are not relationships between the wear intensity of composites with wood and their mechanical property [12]. Unlike the situation where an abrasive process appears, nor hardness neither elastic modulus are representative. The cause is that sliding produces significant modifications of the superficial layer to both triboelements surfaces. The scheme presented in figure 5 is analogue to that for polymers and their composites [8]. Another phenomenon that influences both composite and metallic superficial layers is the transfer process from the metal surface to the softener tribolayer [9]. An important role in the superficial layer structure has the lubricant; which may determine the appearance of adsorbed on chemisorbed layers, on the contact surfaces of triboelements. By their structure, composites with wood and phenol-formaldehyde resins have the capacity of retaining small quantities of lubricant on their surface, for longer sliding periods and thus adsorption processes occur in the same time with absorption processes [10]. Cracking or plasticising stresses Melting or orientated flood thermal and/or oxidant failure composite with wood based on phenol-formaldehyde resins metal transfer and particles including Lubricant influence absorption and adsorption phenomena Polymer or fibbers or adding material transfer oxidation, corrosion metallic counter-element (steel) Physical and mechanical properties abrasion like fine polishing changing of the superficial layer Fig. 5 Changing due to friction in the superficial layers.

4 110 NATIONAL 5. SUPERFICIAL LAYER OF WOODEN COMPOSITES USED IN SLIDING TRIBOSYSTEMS Due to numerous phenomena and processes that occur in the superficial layer of the elements in friction contact, a lot of changing happen. The geometry of surfaces in contact, their structure and composition change and, in the end, characteristic layers are formed, called tribolayers. Rational using of wooden composites for bearings is possible only if a correct evaluation of the physical mechanical properties is made. Technical literature confirms the fact that in sliding bearing construction, composites with wood base and 20% phenol-formaldehyde resins are used under water with oil lubrication conditions [11]. Research was made on tribomodel: roller (steel) shoe (LSD-A composite). LSD-A is a composite made of multilayer phenoplast with beech veneers with 35 % phenol-formaldehyde resins STAS (fig. 6). 1- roller, 2 - shoe, 3 - case, 4 lubricant alimentation. Fig. 6 Experimental tribomodel. The LSD A has a parallel orientation of beech veneers. The testing rig is shown in figure 7. It has the following characteristics: - continuos sped adjustment field rpm; - mechanical loading to the tribomodel from 0 to 40kN; - friction torque applied load and temperature monitoring torque and rotation transducer T30FN200; 2 load transducer C9A Fig. 7 General view of the tribological testing machine. 1 Monitoring system is made of torque transducer 1, force transducer 2, temperature transducer K thermocouple type (NiCr-NiAl) (table 1), dynamic acquisition data system DMCplus (Hottinger- Baldwin ) for torque, rotation speed, load and temperature acquisition device, all connected to a PC. The tribosystem lubrication was with water (η= Ns/m 2 ) and oil-in-water emulsion mixed with sunflower oil and solid soap (η= Ns/m 2 ). Table 1. Transducers characteristics. Torque transducer T30FN200 Maximum torque 200 N.m Nominal rotation speed 3000 rot/min Thermocouple transducer Measuring domain K NiCr-NiAl C Load transducerc9a Maximum load 50 kn Table 2 gives the physical-mechanical characteristics of the LSD-A composite that have been experimentally established [10]. Analysing the testing results, the following appreciation can be done. A longitudinal elastic modular, 10 times smaller than the one for steel, means that LSD-A is an elastic material, and has a good conformity in contact with steel surfaces. The Poisson coefficient is comparable to the one of steel and its value influences the elastic behaviour. Brinell hardness is 10 times smaller than the one of the steel roller, and prevents the appearance of small welding in the contact area. The level of resilience confirms a good shock behaviour. The bearing elastic modulus has a value very close to the longitudinal elastic modulus meaning that the material has a good conformity in contact. Realising the contact between the shoe and the roller s on the entire roller s length can reduce bending deformations. The elastic properties of LSD-A composite can be explained based on phenol-formaldehyde resin s structure that realises the bound between the wooden veneer layers and the cellulose content. The resin obtained by multiple condensation reactions is made by macromolecules, that grow up in three dimensions space and under certain conditions has high elastic properties. Based on molecular theories that describe the statistic behaviour of three-dimensional networks based on single chains properties, we can appreciate that the tests made for rheological characterisation of LSD-A [10] have shown that: - the material has a linear viscous-elasticity; - the existence of remaining deformations for short time loading as well as under constant loading; - hysteresis phenomenon both for bending and traction.

5 NATIONAL 111 Table 2. Physical mechanical characteristics for LSD. Characteristic S.I. Size Longitudinal elasticity modulus MPa ( ).10 4 Bending elasticity modulus MPa ( ).10 4 Transversal elasticity modulus MPa ( ).10 4 Poisson coefficient Braking load in veneers separation plan N Brinell toughness both longitudinal and transversal on the fibber MPa 239 Resilience a) perpendicular fibber on the action direction of the pendulous - specimen without crack - specimen with crack b) parallel fibber with the action direction of the pendulous - specimen with crack Water limit adsorption (10 days maintaining ) a) parallel fibber with the shaft section b) perpendicular fibber on the shaft section Limit degree of inflating in water (10 days maintaining) a) parallel fibber with the shaft section - specimen radial direction - specimen axial direction b) perpendicular fibber on the shaft section - specimen radial direction - specimen axial direction - specimen compression direction (specimen thickness) N.m/m % % As concerning the temperature and thermal conductibility the following appreciation s can be made: - for the couple steel LSD-A the optimum temperature in functioning is 60 o C and the maximum admissible is 80 o C, these values are in good concordance with the temperature of 70 o C, which is the softening point of fat acids; - the optimum temperature is 80 o C, from the material strength point of view, because when the - if temperature rises in the contact area, secondary phenomena may appear; - due to low thermal conductibility of wooden composites, the heat evacuation is difficult to be done and the temperature may suddenly rise to a value higher than 120 o C when the LSD-A structure begins to fail due to the burning of phenol-formaldehyde resin. The tested triboelements surface roughness was within the values the special literature indicates meaning lower than Ra=2-4µm. This value is optimum for the shaft roughness in order to have a lower friction coefficient (roller Ra= µm; shoe Ra= µm). Experimental researches showed a low wear of the triboelements that could not be evaluated, due to water absorption capacity of wooden composites. The low wear of the triboelements is confirmed by the fact that two surfaces with a different crystal lattice have low friction strength and, in time, a low wear. This affirmation is in good concordance with wood theory: friction is the results of altering the superficial forces field, belonging to the two bodies sliding on each other. Two surfaces with identical chemical composition are gripping due to the force field symmetry. In the case of the couple steel LSD-A, the two materials have a complete different structure and the elastic modulus of the composite is ten times smaller than the one for steel. The friction coefficient was within comparable limits to the ones obtained when tribometallic surfaces are in relative motion and there is an oil film between them. The value of the friction coefficient in the water lubrication case, in opened circuit, depends of the water capacity of absorbing and evacuating the heat resulted because of friction between the two surfaces in contact, and of the good conformity of the composite in contact with the metallic surface (fig. 8). When an emulsion is used for lubrication, the value of the friction coefficient (fig. 9) is determined by the presence of sunflower oil and solid soap in the lubricating emulsion. They determine the appearance of then adsorbed films on the surfaces of the two bodies in contact (fig. 10). The optimal molecular structure of lubrication with thin films consists in the length of the hydrocarbon chains with an active radical at one end. The forming of a strong adherent film is influenced by the existence of an active radical in the molecule s structure. The oleaginous plants contain many more saturated fat acids, having a better thermal stability. The firing temperature of the vegetable oils, about 80 o C, is in good correlation to the optimum functioning temperature of the wooden composites. The adsorption phenomenon can be explained based on molecule polarisation.

6 112 NATIONAL Friction coefficient The specimen B3S7AD; B3-Roughness 0.5; S7-Roughness Sliding speed [m/s] Fig. 8 Friction coefficient versus specific load and speed, water lubrication in opened circuit p=0.5mp a p=1mpa p=1.5mp a p=2mpa p=2.5mp Friction coefficient The specimen B6S8, B6-Roughness 0.6; S8-Roughness 1.6 p=0,5mpa p=1.0mpa p=1.5mpa p=2.0mpa p=2.5mpa p=3.0mpa p=3.3mpa Sliding speed (m/s) Fig. 9 Friction coefficient versus specific load and speed, emulsion lubrication in closed circuit. LSD - A apa 3 CH apă a) COOH CH 3 COOH COOH CH3 CH3 COOH Fig. 11 Microscopic analysis of specimen S7. Metal b) Fig. 10 Adsorption and absorption phenomena in the case of steel- LSD A, material couple. Fig. 12 Microscopic analysis of specimen S8.

7 NATIONAL 113 Ffriction coefficient Speciffic load [MPa] The specimen S2 The specimen S5 Sliding speed v=2m/s Fig. 13 Friction coefficient versus specific load and speed, water lubrication in closed circuit. Fig. 14 Microscopic analysis of specimen S2. Fig. 15 Microscopic analysis of specimen S5 The energetic characteristics of the water can be modified with insoluble substances (fat acid, superior aliphatic alcohol, fats considered three-glycerine of fat acids), which in contact with water sit on its surfaces forming monomolecular films. The thickness of the adsorption layer depends of the field intensity given off the metal and the organic substance concentration. The way of the adsorbed film is formed in the case of the couple steel LSD-A is presented in figure 10. Due to the porous structure of the composite, the water gets into the structure capillary, the adsorption phenomena existing in the same time with the absorption ones. A part of the fat acid molecules that go out of the action side of the metallic surface will direct with the polar part to water, the existing water pellicle being the results of its absorption into the composite structure. The microscopic analyse of specimen S7 (fig. 11) and specimen S8 (fig. 12) shows a higher shining degree of the S8 surface. This shows that the local oxidation processes are less intense due to oil presence in the emulsion. The presence of these oils leads to smaller friction coefficients at comparable contact specific loading (fig. 8 and fig. 9). The more intense shining surface indicates smaller superficial changing, both physical and chemical, under the action of mechanical factors. The changing of the surface colour can be an appreciation criterion of the failure degree of the surfaces in contact, due to oxidation processes or due to including processes of the metal particle resulted from the wear process or from different chemical reactions between the composite and the lubricating agent. Figure 13 gives the variation of the friction coefficient; for the specimen S 2, where the wood fibbers are parallel to the transversal section of the shoe and the specimen S 5, where the wood fibbers are perpendicular to the friction surface. Smaller values are for the case when the wood fibber is perpendicular on the friction surface. The water lubrication was in closed circuit with 2m/sec speed. The microscopic analyse showed that the opaqueness degree of the specimen S 2 is higher than the one of S 5 specimen. The opaqueness degree may be correlated with the density degree. Thus, for specimen S 2, where the wood fibbers are parallel to the transversal section of the shoe (fig. 14), the superficial layer became more compact comparing to the one of specimen S 5, where the wood fibbers are perpendicular to the friction surface (fig. 15). This proves that perpendicular fibber has a better toughness in the contact area. The microscopic results explain the difference between the different experimental friction coefficients (fig. 13), and point the influence of the fibber directing on the sliding friction resistance.

8 114 NATIONAL 6. CONCLUSIONS Based on experimental researches and microscopic analysis, the following conclusions can be obtained: - besides the lubrication quality, water is a good cooling agent, water lubrication in open circuit has the advantage of eliminating the heat from the contact area, thermal regime is determined by maintaining the friction coefficient at low limits; - in the case of emulsions, the presence of the organic acids in the sun flower oil and of solid soap, determined a lower friction by forming absorbant films, as a results of the phenomena taking place in the contact area; - the wood fibber direction is important from the friction resistance point of view, and the wood fibbers perpendicular to the friction surface is the best one. REFERENCES 1. Andersson P., Lintula P., 1994, Load carrying capability of water lubricated ceramic journal bearings, (Tribol. Int.), vol. 27, no. 5, pp Beliankin F. P., 1974, Procinosti I deformativnosti sloistâh plasticov, Kiev, Naukova dumka. 3. Bowden F. P., Tabor D. 1964, The Friction and Lubrication of solid, PI - II, Oxford at the Charendon Press. 4. Bratcu O., 1997, Contribuţii la studiul tribosistemelor de alunecare de fricţiune şi antifricţiune, Ph.D, Universitatea Dunărea de Jos Galaţi. 5. Cartier M., 1992, Tribologie des fluides à faible pouvoir lubrifiant (Aplication au frottment en eux), Matheriaux et Technique, France. 6. Caubet J. J., 1964, Theorie et pratiqueindustrielle du frottment, Paris, Technip, Dunot. 7. Crudu I., 1985, On the concept of Tribosystem and Tribomodelling Criterion, Eurotrib 85, Escully, France. 8. Evans D. C., Lancaster J. K., 1979, The wear of Polymers, in Treatise on Materials Science and Technology, vol. 13, Wear, Academic Press inc., New York. 9. Frêne J., Nicolas D., Deguerece B., Berthe D., Godet M., 1990, Lubrification hydrodinamique. Paliers et butées, Edition Eyrolles, Paris, France. 10. Hapenciuc M., Contribuţii la studiul lagărelor greu încărcate, cu cuzineţi din materiale plastice, cu ungere cu apă, Ph.D, Universitatea Dunărea de Jos Galaţi, I1. Iaşcemko V. F., 1966, Procinosti i polzucesti sloistâh plasticov, Kiev, Izd. Naukova dumka. 12. Jinescu V. V., 1979, Propietăţile fizice şi termomecanica materialelor plastice, vol. I, II, Ed.Tehnică, Bucureşti. 13. Krupcinov B. I. et al., 1980, Metodî polucenia i ispolzovannia compozitov na osnove drevesinî dlia detalei uzlov trenia, Vestnik Maşinostroenie, nr Lavielle L., 1991, Polymer-polymer friction: relation with adhesion, Wear, 151, pp Tudor A., 1990, Contactul real al suprafeţelor de frecare, Editura Academiei Române.