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Surface observations of a powder layer during the damage process under particulate lubrication
Wear 297 (2013) 841–848
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Wear journal homepage: www.elsevier.com/locate/wear
Surface observations of a powder layer during the damage process under particulate lubrication Wei Wang n, Xiaojun Liu, Kun Liu Institute of Tribology, Hefei University of Technology, Tunxi Road 193, Hefei, Anhui 230009, China
a r t i c l e i n f o
abstract
Article history: Received 3 April 2012 Received in revised form 13 August 2012 Accepted 15 October 2012 Available online 2 November 2012
Loose powder is used to lubricate the tribopair with the replenishing mechanism. This work analyzes the contact and the damage behavior of the powder layer during powder lubrication. The typical life cycle of a powder layer includes the full powder layer, partial detachment, serious detachment, and complete destruction, which can be concluded from the powder layer images. The carbon and copper content remaining on the surface are analyzed using scanning electron microscopy and energy dispersive X-ray spectroscopy. Layer blistering, partial detachment, delamination, and scuffing, which represent the different forms of damage and deterioration grades, are observed using an optical microscope. Layer blistering, partial detachment, and delamination are the damage forms that occur at earlier stages. Meanwhile, most of the speed difference is accommodated via shearing and sliding of the powder layer. Scuffing, which indicates direct contact and rubbing between the steel surface of the top sample and the copper surface of the bottom sample, is a serious form of damage and is often observed before the full destruction of the powder layer. & 2012 Elsevier B.V. All rights reserved.
Keywords: Graphite Sliding friction Scuffing Solid lubricants Optical microscopy
1. Introduction Solid lubricants can be applied to a tribological interface in various forms, and used successfully in many engineering areas [1–4]. In most modern applications, thin films of solid lubricants are typically deposited on surfaces via advanced vacuum deposition processes to achieve strong bonding and obtain a dense microstructure and uniform thickness [5–8]. Moreover, solid lubricants can strongly bond with a surface using proper adhesives to provide a longer wear life [9–11], and can also be dispersed or impregnated into a composite structure [12–14]. However, the lifetime of most solid lubricants are still limited because of the finite lubricant film thickness. To increase their durability, a self-replenishing or resupply system is needed but is difficult to achieve [15]. Loose powders were also successfully used to lubricate sliding bearing surfaces. Certain powder lubricants have been blended in an aerosol carrier and sprayed directly onto the surfaces [16–19]. The results showed that powder lubricant films provide a lift and separate the bearing surfaces, resulting in a drastically reduced friction coefficient and wear of the tribomaterials. Furthermore, bearing side leakage carries away most of the heat generated by shearing [20]. Heshmat et al. [21] proposed a theoretical model
n
Corresponding author. Tel./fax: þ 86 551 290 1359. E-mail address: [email protected] (W. Wang).
0043-1648/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.10.020
for quasi-hydrodynamic lubrication using particulate matter, and several solutions were obtained. The in situ deposition of boric acid in dry powder form is investigated as a potential, environmentally benign, solid lubricant for sliding metal contacts [22]. In their study, boric acid powder is aerosolized and entrained in a low velocity jet of nitrogen gas, which is directed at a sliding contact in a rotating pin-on-disk tribometer. Friction coefficient below 0.1 can be consistently reached and maintained as long as the powder flow continues. Wear rate is reduced over two orders of magnitude. Many factors, such as the powder properties, the experimental setup, and service conditions affect the characteristics of powder lubrication. Higgs and his colleagues [23–25] conducted a series of tests on a tribometer consisting of simultaneous pellet-on-disk and pad-on-disk sliding contacts, and proposed an asperity-based fractional coverage model for studying the process of lubricant film transfer. Their work showed that the MoS2 pellet actually acted as a self-repairing, self-replenishing, and oil-free lubricant, and found that abrasive wear is the predominant wear mechanism governing the transfer film process. Their analysis model can predict both the friction coefficient at the pad/disk interface and the wear factor of the lubricated pellet/disk sliding contact. McKeague et al. [26,27] presented a theory for predicting the behavior of a powder-lubricated slider bearing that considers the slip of the flow velocity at the boundaries. The formulation includes the boundary roughness and granular temperature. Kimura et al. [28,29] investigated the effect of the components
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and composition of powder lubricants on their insulating and lubricating ability. Results showed that the insulating ability of powder lubricants is higher than that of conventional oil- or water-soluble lubricants and effectively prevents the formation of a scattered chill structure. Drozdov et al. [30,31] applied magnetic powder as a lubricant to the rubbing zone. Gears were placed in a magnetic field so that the lubricant and the magnet formed a single magnetic circuit. Using magnetic powder lubricants enabled the toothed reduction gears to operate satisfactorily under Hertzian contact loads of up to 1100 MPa. Reddy et al. [32,33] conducted a series of experiments to study the effect of a graphite lubricant on surface roughness, grinding force, and specific energy during the grinding of silicon carbide (SiC) materials. Results showed a considerable improvement in the performance of grinding SiC using graphite as a lubricant compared with dry grinding in terms of specific energy requirements, surface roughness, and damage. Wang et al. [34,35] carried out the ring-on-flat experiments that proved the powder properties significantly affect the tribological characteristics of powder lubrication and analyzed the effects of the sliding velocity and normal load. The presence of powder in the rubbing interface is the precondition for powder lubrication. Previous experiments showed that powder could be introduced into the frictional clearance without any special treatment. In these studies, the powders were compacted, and the powder layer was formed under pressure from the tribopair. Several macroscopic phenomena related to the powder layer were reported, while the microscopic process are not well discussed. The process and mechanism of powder layer micro-damage is yet to be elucidated because little research has so far focused on this topic. However, the damage behavior of the compacted powder layer is important to the lubrication characteristics. The current study tries to reveal the state of the powder layer during powder lubrication. It, especially, focuses on the typical microscopic damage and its inherent mechanism, which helps to understand better the process of powder lubrication.
consisted of several steps, namely, mixing the alloy powders (LBC3/Cu10Sn10Pb), spreading the mixture on a carbon steel belt (Thickness 1 mm), sintering (850 1C, 15 min, ammonia decomposition N2 þH2), roller compaction (Diameter 260 mm, double roller, rolling reduction 20%), resintering (820 1C, 15 min, ammonia decomposition N2 þH2), and roller re-compaction. The sheet is cut into 32 32 3 mm square pieces. The surface roughness Ra of the lower sample is 5 mm. The residual porosity of the copper alloy layer is approximately 5%. Before the top sample is brought in contact with the bottom sample, ten grams of loose graphite powder are covered on the bottom sample evenly. These loose powders are then compacted by the normal load, which is applied along the vertical axis of the upper specimen. During the experiments, the upper sample keeps rotated, whereas the lower sample is fixed. Some powder inside the interface may leaked out from the friction interface. Meanwhile, some of the powder remains outside of contact occasionally drawn in the interface along with the movement of the top sample. The average diameter of graphite powder particles is 30 mm. The duration time of every test is 10 min. The environment temperature is 25 1C, and the relative humidity is 75%. The sliding velocity, which is calculated based on the middle line of the annular face, is from 0.2 to 0.8 m/s and the normal load is from 2.94 to 17.64 MPa. The friction torque and normal load are obtained using two force sensors, and the friction coefficient is calculated via conversion based on the structural relationship. The sample temperature is measured in real time using a thermocouple placed at the center of the bottom sample. After the tests, the powder layers on the surface of the bottom sample are observed using a digital camera, an Olympus BH-20 optical microscope and a scanning electron microscope (SEM, JSM-6490LV). The surface elements are analyzed using an energy dispersive X-ray spectrometer (EDS, JSM6490LV), which conducted the area measurement at the same area observed under SEM. The roughness of the friction surface is measured by a Talyor-Hobson-6.
2. Experimental setup
3. Typical life cycle of the powder layer
The tribotester used in the experiments is equipped with a ring-on-flat contact rubbing pair (Fig. 1). The annular top sample, with a 24 mm outside diameter and 16 mm inside diameter, is made of carbon steel, with a hardness of HRC52 and a surface roughness Ra of 0.75 mm. To facilitate powder entrance into the frictional interface, four notches (3 mm) are made along the radial direction, and the contact face corner (1 mm) is rounded. The lower sample is manufactured via powder metallurgy, which
3.1. Four distinct stages
Fig. 1. Schematic diagram of the tribopair and images of the rubbing pairs.
Known from previous studies [34,35], the effects of the sliding velocity and normal load on the lubrication characteristics and powder layer formation are complicated and should be analyzed according to the running conditions of a real application. In Fig. 2, four regions have been divided to indicate the powder layer state and friction characteristics according to variations of service condition. It should be noted that no critical value indicated the transition of each powder layer state. Instead, the transition from one state to another was gradual and was related to real service conditions. Region I is working under high velocity and light load. The friction coefficient is 0.08–0.16 and the full powder layer
Fig. 2. State chart of powder lubrication corresponding to service conditions.
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(Fig. 3a) formed on the surface of the bottom sample. The top sample slides on the powder layer rather than on the bare substrate of the bottom sample, and shearing occurs in the powder layer; hence, the direct contact of the tribopair is prevented. The compact third body powder smoothed the original surface whose roughness Ra reduced from 5 to 0.9 mm. The friction coefficient in Region II is 0.15–0.21, and the temperature is the lowest one due to its low velocity and light load. The friction coefficient in Region III is 0.08–0.12, and the temperature is the highest one due to its high velocity and heavy load. The formation of the powder layer in Region II and Region III is similar. The powder layer starts to deteriorate, and a small portion of the powder layer may lose its adhesion to the bottom sample. At first, the blistering or partial detachment of the powder layer occasionally occurs (Fig. 3b). However, most of the load is still carried by the powder layer and no direct surface contact occurs. Then, serious damage to the powder layer may occur, and a large portion of the powder layer begins to detach from the bottom sample (Fig. 3c). Both the powder layer and the contact surfaces carry the load at the same time. The speed difference is accommodated not only via powder layer shearing but also via sliding between the rubbing surfaces. The surface roughness Ra in Regions II and III are varied from 0.2 to 0.6 mm, which is rougher than that in Region I. It may attribute to the smoothed asperity on the bottom surface. At the final stage (Region IV), the friction coefficient is 0.08–0.21. Although the powder layer is fully destructed, there are little residual powder remaining in the valley of the surface at first (Fig. 3d). Meanwhile, the surface roughness Ra is 0.8–0.9 mm. While the residual powder exhausted, the metal-to-metal contact becomes the
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primary behavior in the friction interface. The surface roughness Ra increased to 5–6 mm while serious wear track was found. Therefore, the heavier load tends to deteriorate the powder layer, which is in accordance with common experience. However, the effect of velocity is quite different, that higher velocity prolongs the life of powder layer. It indicates that higher velocity facilitates the powder to flow into the friction interface. Because of the excellent lubricity of solid lubricant under extreme pressure, the friction coefficient is low at high load while the powder layer is badly broken. Therefore, the analysis of the powder layer is difficult, and how to keep it in good state is a tough challenge. The characteristics of the powder layer are the comprehensive results under the effect of the running conditions, the physical characteristics of the powder and sample surface design. 3.2. Element contents on the wear track Fig. 4 shows the micro observation results on the frictional surface by SEM. The element contents of the same zone as Fig. 4 are analyzed separately using EDS. Table 1 lists the weights and atomic percentages of carbon, oxygen, copper, tin and lead. Fig. 4(a) is the original surface of the bottom sample, which is produced by powder metallurgy. The original surface has the minimum carbon percentage and the maximum copper percentage. During partial detachment, most of the frictional surface is covered by graphite powder (Fig. 4b). Meanwhile, the weight proportion of carbon on the surface is 60.71%, and that of copper is 23.1%. Fig. 4(c) shows that the detachment of the powder layer worsened during the serious detachment period, and the area covered by the powder layer is smaller than that of the opposing
Fig. 3. Typical life cycle of a powder layer: (a) full powder film; (b) partial detachment; (c) serious detachment; and (d) complete destruction.
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Fig. 4. SEM surface images: (a) original surface; (b) partial detachment; (c) serious detachment; and (d) complete destruction.
Table 1 Surface element analysis via EDS. Element Original surface
C O Cu Sn Pb
Partial detachment
Serious detachment
Complete destruction
Wt%
At%
Wt%
At%
Wt%
At%
Wt%
At%
1.18 11.18 59.61 9.33 18.69
5.16 36.70 49.27 4.13 4.74
60.71 7.42 23.10 2.66 4.94
84.90 7.80 6.11 0.38 0.40
37.31 9.61 41.71 4.81 5.12
69.74 13.48 14.74 0.91 0.55
14.36 7.88 63.25 7.89 6.26
42.92 17.68 35.75 2.39 1.09
copper substrate. EDS results show that the weight ratio of carbon is decreased to 37.31%, whereas that of copper is increased to 41.71%. When the powder layer is completely destroyed, the deformed copper covers most of the frictional area (Fig. 4d). The weight ratio of carbon is decreased to 14.36%, whereas that of copper is increased to 63.25%. Therefore, SEM and EDS results confirmed that the tribological characteristics are closely related to the state of the powder layer during powder lubrication. Meanwhile, the lubrication deterioration and the wear increase are usually accompanied by the lack of powder in the friction interface.
4. Typical micro phenomena 4.1. Full powder layer As loose powder is introduced into the rubbing interface, the powder fills the valleys of the rough surface during the early stage. The powder in the valleys is then compacted under the pressure of the counterpart. Together with the asperities, these powders begin to carry the load. More importantly, the
compacted powder reduces the friction between the rubbing surfaces. A powder layer starts to form between the two surfaces as more powder enters the frictional interface. The full powder layer is a fine, compacted, and shell-like layer, which separates the bottom and top samples and prevents direct contact between them. Meanwhile, the internal shearing of the powder layer and the sliding between the powder layer and the top sample accommodate the speed difference of the two surfaces. In Fig. 5(a), the powder layer formed on the surface of bottom sample is integrative and fully covers the substrate surface, although obvious scratches and grooves are observed. Fig. 5(b) shows the microscopic image of an edge of the powder layer. On the left is the powder layer, and the right shows the original surface of the bottom sample. The shell-like powder layer, which prevents the direct contact between the rubbing surfaces, is clearly observed from the edge. The thin, compacted powder layer adheres well with the base material and can last a long time because of replenishment from the loose powder surrounding the tribopair. 4.2. Layer blistering As previously mentioned, the loose powder in the interface between the rotating top sample and the fixed bottom sample is compacted to a thin shell-like layer. No adhesive treatment was applied between the powder layer and the bottom sample in the experiments. Thus, the bond between the powder layer and the bottom sample is weak. Meanwhile, the sliding between the top sample and the powder layer mainly accommodates the speed difference. However, the increase in shearing of the inner powder layer weakens the adhesion between the powder layer and the bottom sample when the running conditions are not optimal. A portion of the shell-like layer detaches from the bottom sample, and a blister layer appears when the top sample is removed (Fig. 6). In the current experiments, layer blistering occurred
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Fig. 5. Images of the full powder layer: (a) scratch traces on the full powder layer, and (b) an edge of the full powder layer.
Fig. 6. Layer blistering of the powder layer: (a) small blister at the edge; (b) large blister; and (c) cluster of blisters.
when the load exceeded 8.82 MPa or the velocity was lower than 0.4 m/s. The powder layer began to detach from the bottom sample as the blisters multiplied (Fig. 6(b) and (c)). In general, layer blistering is the first sign prior to powder layer detachment. When layer blistering occurs, the direct contact between the frictional surfaces is yet to occur. However, necessary actions need to be taken to prevent the detachment of the powder layer. 4.3. Partial detachment Partial detachment means that a portion of the powder layer detaches from the bottom sample. When the state of the powder layer transformed into the partial detachment, the surface of the bottom sample opposes the region where the powder layer has detached. Meanwhile, direct contact and friction between the tribopair is ignorable, and the surface of the bottom sample preserves its original state. Fig. 7 illustrates several typical forms of partial detachment. Fig. 7(a) is the microscopic image of the pitting detachment, showing that a few pits are scattered in a
small area. The obvious feature of the plough detachment (Fig. 7b), which is another form of partial detachment, is the direction of the groove, which coincides with the sliding direction of the friction track. The mechanism for this phenomenon is that some hard particles, either introduced from the outside or resulting from the aggregation of loose powder under the effect of the rotating top sample, plowed the powder layer. Fig. 7(c) shows a large partial detachment of the powder layer at the margin of the friction ring. This indicates that the powder layer detached from the bottom surface, and blistering may have formed prior to detachment. 4.4. Delamination Delamination is another major form of powder layer detachment, in which shearing occurs and delaminates the powder layer into several sub-layers, which can be carried by the rotating top sample or remain on the bottom sample separately. In Fig. 8(a), although a large piece of the powder layer has detached from
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Fig. 7. Partial detachment of the powder layer: (a) pitting detachment; (b) plowing detachment; and (c) partial detachment at the edge.
Fig. 8. Delamination of the powder layer: (a) integrative delamination; and (b) irregular delamination.
bottom sample, the powder layer remaining on the surface is preserved as homogenous and integrative. Different from that observed in Fig. 8(a), the remaining powder as shown in Fig. 8(b) is irregularly distributed in the detachment area. 4.5. Scuffing The steel and copper surfaces meet and rub against each other when the powder layer is detached, eventually resulting in scuffing. Scuffing is another typical form of powder layer damage and is much worse than partial detachment or delamination. Little powder exists in the friction interface, and it is difficult to introduce more powder into the friction interface. At the early stage of scuffing (Fig. 9a), the load carrying and shearing of the powder layer in the frictional interface are the dominant behavior. The frictional track on the copper surface is observed at the area where the powder layer has detached, indicating that the powder supply has become scarce. Moreover, the powder layer remaining on the bottom sample consists of many pieces of the smaller region scattered throughout the friction area. Obvious
cracks at the boundary of these scattered regions are observed. As scuffing proceeds (Fig. 9b) direct contact between the friction surfaces increases, resulting in a long scuffing trace or a large scuffing area. At this stage, the contact surface area is close to the area where powder remains. Moreover, they exert similar effects on the friction characteristics and load carrying capacity. Serious scuffing eventually occurs under severe service conditions (Fig. 9c). Meanwhile, the dominant behavior in the friction interface is the direct contact between the rubbing surfaces, and powder is barely detected in the friction area. Therefore, severe scuffing seriously damages the surface of the tribopair via scratching and wearing.
5. Conclusions This work conducted a series of experiments on loose powder lubrication using a ring-on-flat tester. Utilizing optical microscopy, SEM, and EDS, the state and micro phenomena of powder layer are discovered. The typical life cycle of the powder layer
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Fig. 9. Scuffing of the friction surface: (a) light scuffing; (b) medium scuffing; and (c) serious scuffing.
consists of the full powder layer, partial detachment, serious detachment, and complete destruction. No critical values are available to indicate the state of the powder layer because the transition is gradual. During the transition process, the changing of the powder layer form, the surface element content, and the friction surface roughness closely related to the typical transition state. Due to higher velocity facilitated the powder to flow into the friction interface, powder layer is better at high velocity while load keep constant. Because of the excellent lubricity of solid lubricant under extreme pressure, the friction coefficient is lower at high load while the powder layer exhibits worse. When bonding between the powder layer and the bottom sample weakens, layer blistering and partial detachment occurs at an earlier stage. Meanwhile, the powder layer stays in a good state and lubricates the friction surface well. As the powder layer further detaches, scuffing and serious detachment occur, indicating the onset of serious wearing. Scuffing during powder lubrication should be avoided. Micro damages, such as blistering, delamination, and detachment, appear in every state, but the quantity and degree of deterioration is different at every stage.
Acknowledgment National Natural Science Foundation of China under Grant no. 51005067 and no. 51175136 financially supports the present research. References [1] C. Donnet, A. Erdemir, Solid lubricant coatings: recent developments and future trends, Tribology Letters 17 (2004) 389–397. [2] A. Erdemir, Review of engineered tribological interfaces for improved boundary lubrication, Tribology International 38 (2005) 249–256. [3] W.G. Sawyer, K.D. Freudenberg, P. Bhimaraj, L.S. Schadler, A study on the friction and wear behavior of PTFE filled with alumina nanoparticles, Wear 254 (2003) 573–580.
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Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Surface observations of a powder layer during the damage process under particulate lubrication Wei Wang n, Xiaojun Liu, Kun Liu Institute of Tribology, Hefei University of Technology, Tunxi Road 193, Hefei, Anhui 230009, China
a r t i c l e i n f o
abstract
Article history: Received 3 April 2012 Received in revised form 13 August 2012 Accepted 15 October 2012 Available online 2 November 2012
Loose powder is used to lubricate the tribopair with the replenishing mechanism. This work analyzes the contact and the damage behavior of the powder layer during powder lubrication. The typical life cycle of a powder layer includes the full powder layer, partial detachment, serious detachment, and complete destruction, which can be concluded from the powder layer images. The carbon and copper content remaining on the surface are analyzed using scanning electron microscopy and energy dispersive X-ray spectroscopy. Layer blistering, partial detachment, delamination, and scuffing, which represent the different forms of damage and deterioration grades, are observed using an optical microscope. Layer blistering, partial detachment, and delamination are the damage forms that occur at earlier stages. Meanwhile, most of the speed difference is accommodated via shearing and sliding of the powder layer. Scuffing, which indicates direct contact and rubbing between the steel surface of the top sample and the copper surface of the bottom sample, is a serious form of damage and is often observed before the full destruction of the powder layer. & 2012 Elsevier B.V. All rights reserved.
Keywords: Graphite Sliding friction Scuffing Solid lubricants Optical microscopy
1. Introduction Solid lubricants can be applied to a tribological interface in various forms, and used successfully in many engineering areas [1–4]. In most modern applications, thin films of solid lubricants are typically deposited on surfaces via advanced vacuum deposition processes to achieve strong bonding and obtain a dense microstructure and uniform thickness [5–8]. Moreover, solid lubricants can strongly bond with a surface using proper adhesives to provide a longer wear life [9–11], and can also be dispersed or impregnated into a composite structure [12–14]. However, the lifetime of most solid lubricants are still limited because of the finite lubricant film thickness. To increase their durability, a self-replenishing or resupply system is needed but is difficult to achieve [15]. Loose powders were also successfully used to lubricate sliding bearing surfaces. Certain powder lubricants have been blended in an aerosol carrier and sprayed directly onto the surfaces [16–19]. The results showed that powder lubricant films provide a lift and separate the bearing surfaces, resulting in a drastically reduced friction coefficient and wear of the tribomaterials. Furthermore, bearing side leakage carries away most of the heat generated by shearing [20]. Heshmat et al. [21] proposed a theoretical model
n
Corresponding author. Tel./fax: þ 86 551 290 1359. E-mail address: [email protected] (W. Wang).
0043-1648/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.10.020
for quasi-hydrodynamic lubrication using particulate matter, and several solutions were obtained. The in situ deposition of boric acid in dry powder form is investigated as a potential, environmentally benign, solid lubricant for sliding metal contacts [22]. In their study, boric acid powder is aerosolized and entrained in a low velocity jet of nitrogen gas, which is directed at a sliding contact in a rotating pin-on-disk tribometer. Friction coefficient below 0.1 can be consistently reached and maintained as long as the powder flow continues. Wear rate is reduced over two orders of magnitude. Many factors, such as the powder properties, the experimental setup, and service conditions affect the characteristics of powder lubrication. Higgs and his colleagues [23–25] conducted a series of tests on a tribometer consisting of simultaneous pellet-on-disk and pad-on-disk sliding contacts, and proposed an asperity-based fractional coverage model for studying the process of lubricant film transfer. Their work showed that the MoS2 pellet actually acted as a self-repairing, self-replenishing, and oil-free lubricant, and found that abrasive wear is the predominant wear mechanism governing the transfer film process. Their analysis model can predict both the friction coefficient at the pad/disk interface and the wear factor of the lubricated pellet/disk sliding contact. McKeague et al. [26,27] presented a theory for predicting the behavior of a powder-lubricated slider bearing that considers the slip of the flow velocity at the boundaries. The formulation includes the boundary roughness and granular temperature. Kimura et al. [28,29] investigated the effect of the components
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and composition of powder lubricants on their insulating and lubricating ability. Results showed that the insulating ability of powder lubricants is higher than that of conventional oil- or water-soluble lubricants and effectively prevents the formation of a scattered chill structure. Drozdov et al. [30,31] applied magnetic powder as a lubricant to the rubbing zone. Gears were placed in a magnetic field so that the lubricant and the magnet formed a single magnetic circuit. Using magnetic powder lubricants enabled the toothed reduction gears to operate satisfactorily under Hertzian contact loads of up to 1100 MPa. Reddy et al. [32,33] conducted a series of experiments to study the effect of a graphite lubricant on surface roughness, grinding force, and specific energy during the grinding of silicon carbide (SiC) materials. Results showed a considerable improvement in the performance of grinding SiC using graphite as a lubricant compared with dry grinding in terms of specific energy requirements, surface roughness, and damage. Wang et al. [34,35] carried out the ring-on-flat experiments that proved the powder properties significantly affect the tribological characteristics of powder lubrication and analyzed the effects of the sliding velocity and normal load. The presence of powder in the rubbing interface is the precondition for powder lubrication. Previous experiments showed that powder could be introduced into the frictional clearance without any special treatment. In these studies, the powders were compacted, and the powder layer was formed under pressure from the tribopair. Several macroscopic phenomena related to the powder layer were reported, while the microscopic process are not well discussed. The process and mechanism of powder layer micro-damage is yet to be elucidated because little research has so far focused on this topic. However, the damage behavior of the compacted powder layer is important to the lubrication characteristics. The current study tries to reveal the state of the powder layer during powder lubrication. It, especially, focuses on the typical microscopic damage and its inherent mechanism, which helps to understand better the process of powder lubrication.
consisted of several steps, namely, mixing the alloy powders (LBC3/Cu10Sn10Pb), spreading the mixture on a carbon steel belt (Thickness 1 mm), sintering (850 1C, 15 min, ammonia decomposition N2 þH2), roller compaction (Diameter 260 mm, double roller, rolling reduction 20%), resintering (820 1C, 15 min, ammonia decomposition N2 þH2), and roller re-compaction. The sheet is cut into 32 32 3 mm square pieces. The surface roughness Ra of the lower sample is 5 mm. The residual porosity of the copper alloy layer is approximately 5%. Before the top sample is brought in contact with the bottom sample, ten grams of loose graphite powder are covered on the bottom sample evenly. These loose powders are then compacted by the normal load, which is applied along the vertical axis of the upper specimen. During the experiments, the upper sample keeps rotated, whereas the lower sample is fixed. Some powder inside the interface may leaked out from the friction interface. Meanwhile, some of the powder remains outside of contact occasionally drawn in the interface along with the movement of the top sample. The average diameter of graphite powder particles is 30 mm. The duration time of every test is 10 min. The environment temperature is 25 1C, and the relative humidity is 75%. The sliding velocity, which is calculated based on the middle line of the annular face, is from 0.2 to 0.8 m/s and the normal load is from 2.94 to 17.64 MPa. The friction torque and normal load are obtained using two force sensors, and the friction coefficient is calculated via conversion based on the structural relationship. The sample temperature is measured in real time using a thermocouple placed at the center of the bottom sample. After the tests, the powder layers on the surface of the bottom sample are observed using a digital camera, an Olympus BH-20 optical microscope and a scanning electron microscope (SEM, JSM-6490LV). The surface elements are analyzed using an energy dispersive X-ray spectrometer (EDS, JSM6490LV), which conducted the area measurement at the same area observed under SEM. The roughness of the friction surface is measured by a Talyor-Hobson-6.
2. Experimental setup
3. Typical life cycle of the powder layer
The tribotester used in the experiments is equipped with a ring-on-flat contact rubbing pair (Fig. 1). The annular top sample, with a 24 mm outside diameter and 16 mm inside diameter, is made of carbon steel, with a hardness of HRC52 and a surface roughness Ra of 0.75 mm. To facilitate powder entrance into the frictional interface, four notches (3 mm) are made along the radial direction, and the contact face corner (1 mm) is rounded. The lower sample is manufactured via powder metallurgy, which
3.1. Four distinct stages
Fig. 1. Schematic diagram of the tribopair and images of the rubbing pairs.
Known from previous studies [34,35], the effects of the sliding velocity and normal load on the lubrication characteristics and powder layer formation are complicated and should be analyzed according to the running conditions of a real application. In Fig. 2, four regions have been divided to indicate the powder layer state and friction characteristics according to variations of service condition. It should be noted that no critical value indicated the transition of each powder layer state. Instead, the transition from one state to another was gradual and was related to real service conditions. Region I is working under high velocity and light load. The friction coefficient is 0.08–0.16 and the full powder layer
Fig. 2. State chart of powder lubrication corresponding to service conditions.
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(Fig. 3a) formed on the surface of the bottom sample. The top sample slides on the powder layer rather than on the bare substrate of the bottom sample, and shearing occurs in the powder layer; hence, the direct contact of the tribopair is prevented. The compact third body powder smoothed the original surface whose roughness Ra reduced from 5 to 0.9 mm. The friction coefficient in Region II is 0.15–0.21, and the temperature is the lowest one due to its low velocity and light load. The friction coefficient in Region III is 0.08–0.12, and the temperature is the highest one due to its high velocity and heavy load. The formation of the powder layer in Region II and Region III is similar. The powder layer starts to deteriorate, and a small portion of the powder layer may lose its adhesion to the bottom sample. At first, the blistering or partial detachment of the powder layer occasionally occurs (Fig. 3b). However, most of the load is still carried by the powder layer and no direct surface contact occurs. Then, serious damage to the powder layer may occur, and a large portion of the powder layer begins to detach from the bottom sample (Fig. 3c). Both the powder layer and the contact surfaces carry the load at the same time. The speed difference is accommodated not only via powder layer shearing but also via sliding between the rubbing surfaces. The surface roughness Ra in Regions II and III are varied from 0.2 to 0.6 mm, which is rougher than that in Region I. It may attribute to the smoothed asperity on the bottom surface. At the final stage (Region IV), the friction coefficient is 0.08–0.21. Although the powder layer is fully destructed, there are little residual powder remaining in the valley of the surface at first (Fig. 3d). Meanwhile, the surface roughness Ra is 0.8–0.9 mm. While the residual powder exhausted, the metal-to-metal contact becomes the
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primary behavior in the friction interface. The surface roughness Ra increased to 5–6 mm while serious wear track was found. Therefore, the heavier load tends to deteriorate the powder layer, which is in accordance with common experience. However, the effect of velocity is quite different, that higher velocity prolongs the life of powder layer. It indicates that higher velocity facilitates the powder to flow into the friction interface. Because of the excellent lubricity of solid lubricant under extreme pressure, the friction coefficient is low at high load while the powder layer is badly broken. Therefore, the analysis of the powder layer is difficult, and how to keep it in good state is a tough challenge. The characteristics of the powder layer are the comprehensive results under the effect of the running conditions, the physical characteristics of the powder and sample surface design. 3.2. Element contents on the wear track Fig. 4 shows the micro observation results on the frictional surface by SEM. The element contents of the same zone as Fig. 4 are analyzed separately using EDS. Table 1 lists the weights and atomic percentages of carbon, oxygen, copper, tin and lead. Fig. 4(a) is the original surface of the bottom sample, which is produced by powder metallurgy. The original surface has the minimum carbon percentage and the maximum copper percentage. During partial detachment, most of the frictional surface is covered by graphite powder (Fig. 4b). Meanwhile, the weight proportion of carbon on the surface is 60.71%, and that of copper is 23.1%. Fig. 4(c) shows that the detachment of the powder layer worsened during the serious detachment period, and the area covered by the powder layer is smaller than that of the opposing
Fig. 3. Typical life cycle of a powder layer: (a) full powder film; (b) partial detachment; (c) serious detachment; and (d) complete destruction.
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Fig. 4. SEM surface images: (a) original surface; (b) partial detachment; (c) serious detachment; and (d) complete destruction.
Table 1 Surface element analysis via EDS. Element Original surface
C O Cu Sn Pb
Partial detachment
Serious detachment
Complete destruction
Wt%
At%
Wt%
At%
Wt%
At%
Wt%
At%
1.18 11.18 59.61 9.33 18.69
5.16 36.70 49.27 4.13 4.74
60.71 7.42 23.10 2.66 4.94
84.90 7.80 6.11 0.38 0.40
37.31 9.61 41.71 4.81 5.12
69.74 13.48 14.74 0.91 0.55
14.36 7.88 63.25 7.89 6.26
42.92 17.68 35.75 2.39 1.09
copper substrate. EDS results show that the weight ratio of carbon is decreased to 37.31%, whereas that of copper is increased to 41.71%. When the powder layer is completely destroyed, the deformed copper covers most of the frictional area (Fig. 4d). The weight ratio of carbon is decreased to 14.36%, whereas that of copper is increased to 63.25%. Therefore, SEM and EDS results confirmed that the tribological characteristics are closely related to the state of the powder layer during powder lubrication. Meanwhile, the lubrication deterioration and the wear increase are usually accompanied by the lack of powder in the friction interface.
4. Typical micro phenomena 4.1. Full powder layer As loose powder is introduced into the rubbing interface, the powder fills the valleys of the rough surface during the early stage. The powder in the valleys is then compacted under the pressure of the counterpart. Together with the asperities, these powders begin to carry the load. More importantly, the
compacted powder reduces the friction between the rubbing surfaces. A powder layer starts to form between the two surfaces as more powder enters the frictional interface. The full powder layer is a fine, compacted, and shell-like layer, which separates the bottom and top samples and prevents direct contact between them. Meanwhile, the internal shearing of the powder layer and the sliding between the powder layer and the top sample accommodate the speed difference of the two surfaces. In Fig. 5(a), the powder layer formed on the surface of bottom sample is integrative and fully covers the substrate surface, although obvious scratches and grooves are observed. Fig. 5(b) shows the microscopic image of an edge of the powder layer. On the left is the powder layer, and the right shows the original surface of the bottom sample. The shell-like powder layer, which prevents the direct contact between the rubbing surfaces, is clearly observed from the edge. The thin, compacted powder layer adheres well with the base material and can last a long time because of replenishment from the loose powder surrounding the tribopair. 4.2. Layer blistering As previously mentioned, the loose powder in the interface between the rotating top sample and the fixed bottom sample is compacted to a thin shell-like layer. No adhesive treatment was applied between the powder layer and the bottom sample in the experiments. Thus, the bond between the powder layer and the bottom sample is weak. Meanwhile, the sliding between the top sample and the powder layer mainly accommodates the speed difference. However, the increase in shearing of the inner powder layer weakens the adhesion between the powder layer and the bottom sample when the running conditions are not optimal. A portion of the shell-like layer detaches from the bottom sample, and a blister layer appears when the top sample is removed (Fig. 6). In the current experiments, layer blistering occurred
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Fig. 5. Images of the full powder layer: (a) scratch traces on the full powder layer, and (b) an edge of the full powder layer.
Fig. 6. Layer blistering of the powder layer: (a) small blister at the edge; (b) large blister; and (c) cluster of blisters.
when the load exceeded 8.82 MPa or the velocity was lower than 0.4 m/s. The powder layer began to detach from the bottom sample as the blisters multiplied (Fig. 6(b) and (c)). In general, layer blistering is the first sign prior to powder layer detachment. When layer blistering occurs, the direct contact between the frictional surfaces is yet to occur. However, necessary actions need to be taken to prevent the detachment of the powder layer. 4.3. Partial detachment Partial detachment means that a portion of the powder layer detaches from the bottom sample. When the state of the powder layer transformed into the partial detachment, the surface of the bottom sample opposes the region where the powder layer has detached. Meanwhile, direct contact and friction between the tribopair is ignorable, and the surface of the bottom sample preserves its original state. Fig. 7 illustrates several typical forms of partial detachment. Fig. 7(a) is the microscopic image of the pitting detachment, showing that a few pits are scattered in a
small area. The obvious feature of the plough detachment (Fig. 7b), which is another form of partial detachment, is the direction of the groove, which coincides with the sliding direction of the friction track. The mechanism for this phenomenon is that some hard particles, either introduced from the outside or resulting from the aggregation of loose powder under the effect of the rotating top sample, plowed the powder layer. Fig. 7(c) shows a large partial detachment of the powder layer at the margin of the friction ring. This indicates that the powder layer detached from the bottom surface, and blistering may have formed prior to detachment. 4.4. Delamination Delamination is another major form of powder layer detachment, in which shearing occurs and delaminates the powder layer into several sub-layers, which can be carried by the rotating top sample or remain on the bottom sample separately. In Fig. 8(a), although a large piece of the powder layer has detached from
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Fig. 7. Partial detachment of the powder layer: (a) pitting detachment; (b) plowing detachment; and (c) partial detachment at the edge.
Fig. 8. Delamination of the powder layer: (a) integrative delamination; and (b) irregular delamination.
bottom sample, the powder layer remaining on the surface is preserved as homogenous and integrative. Different from that observed in Fig. 8(a), the remaining powder as shown in Fig. 8(b) is irregularly distributed in the detachment area. 4.5. Scuffing The steel and copper surfaces meet and rub against each other when the powder layer is detached, eventually resulting in scuffing. Scuffing is another typical form of powder layer damage and is much worse than partial detachment or delamination. Little powder exists in the friction interface, and it is difficult to introduce more powder into the friction interface. At the early stage of scuffing (Fig. 9a), the load carrying and shearing of the powder layer in the frictional interface are the dominant behavior. The frictional track on the copper surface is observed at the area where the powder layer has detached, indicating that the powder supply has become scarce. Moreover, the powder layer remaining on the bottom sample consists of many pieces of the smaller region scattered throughout the friction area. Obvious
cracks at the boundary of these scattered regions are observed. As scuffing proceeds (Fig. 9b) direct contact between the friction surfaces increases, resulting in a long scuffing trace or a large scuffing area. At this stage, the contact surface area is close to the area where powder remains. Moreover, they exert similar effects on the friction characteristics and load carrying capacity. Serious scuffing eventually occurs under severe service conditions (Fig. 9c). Meanwhile, the dominant behavior in the friction interface is the direct contact between the rubbing surfaces, and powder is barely detected in the friction area. Therefore, severe scuffing seriously damages the surface of the tribopair via scratching and wearing.
5. Conclusions This work conducted a series of experiments on loose powder lubrication using a ring-on-flat tester. Utilizing optical microscopy, SEM, and EDS, the state and micro phenomena of powder layer are discovered. The typical life cycle of the powder layer
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Fig. 9. Scuffing of the friction surface: (a) light scuffing; (b) medium scuffing; and (c) serious scuffing.
consists of the full powder layer, partial detachment, serious detachment, and complete destruction. No critical values are available to indicate the state of the powder layer because the transition is gradual. During the transition process, the changing of the powder layer form, the surface element content, and the friction surface roughness closely related to the typical transition state. Due to higher velocity facilitated the powder to flow into the friction interface, powder layer is better at high velocity while load keep constant. Because of the excellent lubricity of solid lubricant under extreme pressure, the friction coefficient is lower at high load while the powder layer exhibits worse. When bonding between the powder layer and the bottom sample weakens, layer blistering and partial detachment occurs at an earlier stage. Meanwhile, the powder layer stays in a good state and lubricates the friction surface well. As the powder layer further detaches, scuffing and serious detachment occur, indicating the onset of serious wearing. Scuffing during powder lubrication should be avoided. Micro damages, such as blistering, delamination, and detachment, appear in every state, but the quantity and degree of deterioration is different at every stage.
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