Effect of relative humidity on the unlubricated wear of metals

Effect of relative humidity on the unlubricated wear of metals

Wear 260 (2006) 720–727 Effect of relative humidity on the unlubricated wear of metals W.Y.H. Liew ∗ School of Engineering & Information Technology, ...

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Wear 260 (2006) 720–727

Effect of relative humidity on the unlubricated wear of metals W.Y.H. Liew ∗ School of Engineering & Information Technology, Universiti Malaysia Sabah, Locked Bag 2073, Kota Kinabalu, Sabah 88999, Malaysia Received 1 December 2004; received in revised form 6 April 2005; accepted 6 April 2005 Available online 23 May 2005

Abstract Experiments were carried out to investigate the effect of humidity on the wear behaviour of same metal combination, i.e. steel–steel and aluminium–aluminium. The sliding wear of steel was found to increase by nearly 1.5 orders of magnitude when the relative humidity (RH) of the surrounding air decreased from 80 to 28% RH. At low humidity, both delamination and adhesion wear occurred. At high humidity levels, both delamination and adhesion wear took place at a relative small scale and the frictional force was considerably lower than that obtained at lower humidity levels. It is proposed that the low wear occurring at high humidity levels is due to the inhibition of these wear mechanisms by the formation of interfacial layers, possibly iron hydroxide and ferri-oxide-hydrates, and the adsorption of water on the worn surface in addition to the normal atmospheric oxidation. Increasing the humidity from 28 to 80% RH increased the wear rate of aluminium by nearly half an order of magnitude. It is proposed that at higher humidity levels, water vapour adsorbs on both the freshly created surface and wear debris generated and therefore the wear debris egresses easily from the contact area without adhering to the parent surfaces. Lack of adhering wear debris exposed the worn surfaces to metal–metal interaction. © 2005 Published by Elsevier B.V. Keywords: Wear; Friction; Humidity; Steel; Aluminium

1. Introduction It is known that in practical applications, the variations in humidity affect the wear rate of metals. The ambient humidity can cover almost the whole range from 0 to 100% RH, and any significant influence of humidity on the wear of metals is important. It has been widely reported that steel is sensitive to the surrounding atmosphere. Klaffke [1] found that the wear of steel increased by a factor of 2 when the humidity was decreased from 100 to 15% RH. Further reducing the humidity from 15 to 3% RH resulted in an increase in wear by a factor of 4. Oh et al. [2] have investigated the effect of humidity on the dry sliding of various types of carbon steel and found that severe wear, two orders of magnitude higher than the mild wear, occurred at relative humidity below 45% RH. While the wear of all steels was essentially the same in the mild wear region, the alloy with higher content of ferrite exhibited lower wear in the severe wear region. They concluded ∗

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that the inclusion had high tendency to react with the oxygen to form a protective oxide film, and therefore increasing the content of the inclusion would result in a reduction in wear. Bregliozzi et al. [3] observed that decreasing the humidity in the wear test of stainless steel resulted in an increase in the adhesion between the contact surfaces leading to an increase in the wear and friction coefficient. In another study on the sliding of stainless steel at loads of lower than 2000 ␮N, Bregliozzi et al. [4] found the coefficient of friction increased with the humidity. A converse of this situation was observed at a higher load of 2 N, where both the wear and coefficient of friction reduced significantly with an increase in the humidity. The low wear rate occurring at high humidity levels could be attributed to the formation of interfacial layers such iron hydroxide and ferri-oxide-hydrates and the adsorption of water on the worn surface [5,6]. While some authors found that wear decreased with humidity, others found the opposite, wear increased with humidity [7,8]. Endo and Goto [8] found that water vapour had no deleterious effect on wear of mild steel when the environment did not consist of oxygen but only water vapour. It had

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been reported that the sliding speed had significant influence on the effect of humidity on wear of steel [9]. At low speed, the wear increased with a decrease in humidity. A contrast of this was observed at high sliding speed, where the wear increased with humidity. Not many papers have reported on the influence of humidity on the wear of aluminium. Several workers [6,10] found that the fretting wear of pure aluminium increased with humidity. In this study, experiments have been carried out to investigate the effect of humidity on the wear of steel and aluminium sliding against their own materials. The frictional force and the worn surface were examined to gain a better understanding of the effect of humidity on the wear of these alloys.

2. Experiment procedure The pin-on-disc wear tester is illustrated in Fig. 1. The test pin, fixed to the load arm, was loaded against the rotating disc. A linear voltage displacement transducer (LVDT) was used to detect the vertical displacement of the arm and consequently the total wear of the pin and disc. The friction force was measured continuously by a load cell mounted on a rigid frame; the value reported corresponds to the end of the test. A dead weight was loaded at the end of the balanced arm to produced a nominal load of 5 N at the contact. The arm was connected to a dashpot damping system to reduce any possible dynamic loading. A one-off built environment unit was used to produce a continual blast of air stream with controlled temperature and humidity. The temperature of the air leaving the water cascade can be controlled by adjusting electric heater. A nozzle was used to direct the air stream over the contact area of the pin and disc. A supply air sensor, located in the exit point of the nozzle, detects the temperature and humidity of the air stream. The sensor feeds data back to the controlling microprocessor which maintains the temperature within ±2 K and the relative humidity within ±3% RH. The reported values are based on the measurements of the temperature–humidity probe located between the nozzle and the pin-disc contact. The distance between the nozzle and the text specimens were kept at 60 mm. Prior to testing, the specimens were placed un-

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Fig. 2. Schematic diagram of the segment of the pin. r, Radius of the hemisphere pin; h, wear height loss; c, pin scar diameter.

der the flow of air at the humidity set for each test for 30 min. The temperature of the air was set at 23 ◦ C throughout the tests. The pins fixed to the load arm and the disc rotated below it such that the speed at the contact point was maintained at 6 m/min for a distance of 500 m. Test materials used in this study were UIC860A rail steel (0.26 wt.% Si, 0.96 wt.% Mn, 0.78 wt.% C, 0.005 wt.% Mo, 0.023 wt.% S, 0.027 wt.% P, balance Fe) and BS1474 alumiuniun (0.1 wt.% Cu, 0.1 wt.% Zn, 0.5 wt.% Fe, 0.004 wt.% Mg, 0.3 wt.% Cr, 0.6 wt.% Si, 0.4 wt.% Mn, balance Al). The hardness of the steel and aluminium are 330 and 47 HV, respectively. The pins were 6 mm in diameter, each with a hemisphere tip of radius of 5 mm, and the disc were 80 mm in diameter and 8 mm thick. The pin and the disc surfaces were ground to a mean roughness value (Ra ) of less than 0.1 ␮m. The specific wear rate was calculated as the total volume removed from the pin per unit sliding distance and load. The volumetric wear of the pins was calculated from the values of the scar diameter, assuming a perfectly hemisphere original tip geometry.   h 2 Volumetric wear of the pin, V = πh r − (1) 3 where r is the radius of the hemisphere pin and h the wear height loss as illustrated in Fig. 2. h is related to r and c the pin scar diameter by 1 1/2 h = r − (4r 2 − c2 ) 2

Fig. 1. Schematic diagram of the pin-on-disc wear machine.

(2)

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Fig. 3. The effect of humidity on the wear rate of steel and aluminium pins. Test conditions were as follows: velocity of 6 m/min, nominal load of 5 N and sliding distance of 500 m.

3. Results and discussion

Fig. 5. The effect of humidity on the pin roughness. For the steel case, there is a clear tendency for surface roughening to increase as the humidity is decreased. No relation is found between the roughness of the worn surface of the aluminium pin and the humidity.

3.1. Steel—results An increase in the humidity from 28 to 80% RH reduced the wear rate of steel pin by nearly 1.5 orders of magnitude (Fig. 3). At the sliding distance of 500 m, the corresponding coefficient of friction at the lower humidity is about twice of that at higher humidity. A correlation was found between the wear rate, coefficient of friction and surface roughness; higher wear loss corresponds to higher coefficient of friction and rougher surface (Figs. 3–5). At the humidity of 62 and 80% RH, rapid wear and the fluctuation of the friction force took place in initial stage of sliding (Fig. 6(a and b)). This followed by a prevailing steady state friction and low wear rate which was not seen in the tests carried out at lower humidity levels (Fig. 7(a and b)). The prevailing low frictional forces obtained at high humidity levels indicates the presence of lubricant films at the worn surfaces. The fluctuation of the friction force and high wear rate during the initial

Fig. 4. Variation of coefficient of friction with humidity. For steel case, the coefficient of friction at the lower humidity is about twice of that at higher humidity. No relation was found between the humidity and the coefficient of friction obtained in the sliding of aluminium.

Fig. 6. Variation of the displacement (a) and frictional force (b) during sliding of steel at 80% RH.

Fig. 7. Variation of the displacement (a) and frictional force (b) during sliding of steel at 28% RH.

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Fig. 8. Scanning electron micrographs of the worn surface of steel pin after sliding at 28% RH (a) and 80% RH (b).

sliding of steel pin on a steel disc reflects the nature of the running-in process for this particular material combination. Running-in process indicates a severe to mild wear transition. Severe wear is inevitable in the early stage of sliding when the substrate hardness of the steel is less than critical value to support an oxide film. Initial running-in process leads to the establishment of a equilibrium surface condition where the critical hardness is achieved [11]. Fig. 8 show a SEM image of the worn surfaces of the pin tested at different humidity conditions. At low humidity of 28% RH, numerous amount of fine cavities formed on the pin worn surface and flake-like delamination wear occurred mainly at the “ridges” (Fig. 8(a)). At 80% RH, delamination wear took place at a much smaller scale and a smoother surface was generated (Fig. 8(b)). Fig. 9 shows a crack of about 110 ␮m long and near continuum formed at the subsurface of the pin (about 10 ␮m from the surface) tested at 28% RH. It is possible that high frictional traction caused an increase in the plastic shear strain accumulated in the subsurface, leading to the formation of cracks along the sliding direction which

Fig. 9. Cross-section of the steel pin tested at 28% RH. Sliding direction left to right. High frictional traction could have caused cracks to form along the sliding direction before propagating to the surface.

eventually propagated to the surface to release flake-like debris from the surface [12,13]. The pin tested at 28% RH were sectioned and the hardness of the subsurface of the pin tested was measured. It was found that the cracked (delamination wear) region (as indicated by ‘A’ in Fig. 10) was plastically deformed, resulting in an increase in the hardness to 1040 HV. This hardness value was much higher than that measured at the subsurface below the “valley” region (as indicated by ‘B’ in Fig. 10) which was found to be less than 450 HV. 3.2. Steel—discussion Oxygen can reduce adhesive wear and friction force by virtue of its ability to form a protective oxide film on the steel worn surfaces [11,14–16]. In particular, it was found that in dry sliding of carbon steels, when the oxide film melted, transition from mild oxidation wear to severe wear (delamination and adhesion wear) took place [14,16]. The presence of water vapour influences both the rate of oxidation and the type of oxides formed. Goto and Buckley [6] have investigated the influence of humidity on the wear of various type

Fig. 10. Hardness profile of steel pin after sliding at 28% RH. ‘A’ is the cracked (delamination wear) region. ‘B’ is the “valley” region.

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of metals and found that the presence of water vapour can interfere the adsorption of oxygen, leading to reduction in the rate of oxidation and thus an increase in wear. On the other hand, Bregliozzi et al. [4] found that in unlubricated sliding of stainless steel, both the wear and coefficient of friction reduced significantly despite that the oxidation rate reduced with an increase in the humidity. Results obtained by Baets et al. [5] indicated that increasing the humidity reduced the oxidation rate on the steel worn surface, but on the other hand promoted other types of oxidation reactions which form iron hydroxide and ferri-oxide-hydrates. These authors concluded that these reactions accounted for the low wear and friction force at high humidity levels. The formation of iron oxide and iron hydroxide oxide has been reported in previous study on the sliding wear of steel, where this mechanism had been suggested as a reason for the mild wear occurring at high humidity [17]. Another possible explanation for the decrease in the wear rates with increasing humidity is that the moisture that is adsorbed physically on the worn surface acts in a protective manner so as to prevent direct contact between the surfaces [6]. The moisture can also mix with other contaminating molecules to form an electrolyte on the metal surface causing electrochemical corrosion in addition to the normal atmospheric oxidation [5]. The mild wear occurring at high humidity levels could be due to the formation of surface films as discussed above which always led to a reduction in the frictional force. Severe adhesion easily occurs on nascent surfaces and this phenomenon normally gives rise to high frictional force [14]. At humidity levels of 50% RH and below, coefficient of friction of greater than 1.3 was obtained. This value was considerably higher than those obtained at 63 and 80% RH (which found to be 0.75 at both humidity levels). At humidity levels of 50% RH and below, due to the lack of protective films formed on the worn surface, adhesive wear occurred. The rupture of the existing adhesive bridges caused the liberation of small debris and thus the formation of fine cavities on the worn surface of the pin (Fig. 8(a)). This wear mechanism could have resulted in a high coefficient of friction measured at low humidity levels. Major contributions have been made to the fundamental understanding of the unlubricated wear of metals. Archard and Hirst [18] examined the wear of a wide range of materials and classified their observations in terms of two wear mechanism, i.e. severe and mild wear. Transition from mild wear to the severe wear was encountered and this took place when there was a change in surface conditions such as breakdown of protective films on the worn surface. Severe wear was characterized by a high wear rate, marked surface roughening with heavy subsurface deformation and large metallic debris particles. In mild wear, the wear rate was about two orders of magnitude lower than that in the severe wear and was accompanied by a smoothing of the surface, with little or no subsurface deformation and fine debris particles. Several researchers [16,19] have carried out an intensive series of wear tests on various type of steel alloys and proposed that the wear condition can be classified as severe if the specific wear rate

is higher than 2 × 10−8 mm3 /(N mm). Wang et al. [16] found that in the severe wear regime, both adhesion and delamination wear occurred and these wear mechanisms gave rise to wear two orders of magnitude higher than mild wear. In this study, the wear took place at 28% RH can be considered as severe wear as both delamination and adhesion wear occurred, and a high specific wear rate of 2.4 × 10−7 mm3 /(N mm) was obtained. At 62 and 80% RH, these wear mechanisms occurred at a much smaller scale and the specific wear rate which found to be 6.5 × 10−9 and 5.4 × 10−9 mm3 /(N mm), respectively, were about 1.5 orders of magnitude lower than that obtained at 28% RH. Therefore, the wear taking place at these humidity levels can be considered as mild wear. 3.3. Aluminium—results Increasing the humidity from 28 to 80% RH increased the wear rate of aluminium by nearly half an order of magnitude (Fig. 3). The variation of the wear rate obtained at each humidity level was less than 15% from the mean value. No relation was found between the roughness of the worn surface of the pin, the wear rate and the coefficient of friction (Figs. 3–5). The wear debris showed higher tendency to adhere on the pin surface as the humidity was decreased. At the 28% RH, the worn surface of the pin was observed to be largely covered by a flattened island of wear debris (Fig. 11(a)). Much lesser amount of wear debris was found to adhere on the pin tested at 80% RH (Fig. 11(b)). The wear debris which had been deformed and sheared (Fig. 12), had a lower roughness value than the nascent worn surface. However, the wear debris piled up on the worn surface may form large pits (Fig. 11(a)) that gives rise to high roughness value. Therefore, the wear debris adhering on the worn surface may result in an increase or decrease in the roughness value, depending on how it is being piled up on the surface. Optical examination reveals that the worn surface tested at 28% RH consists of three main regions, i.e. grooved metallic looking region, bright metallic raised plateau and debris layer (Fig. 13). Figs. 14(a) and 15(a) show the vertical displacement of the pin with sliding distance. The negative displacement indicates that the pin is being lifted up from the track due to accumulation of metal debris at the pin surface. The removal of the debris caused a sudden change from negative to positive displacement. The above repeating processes were more pronounced at low humidity. At the low humidity of 28% RH, in the initial stage of sliding, the pin was lifted up above the starting contact point for a distance of about 70 m. At the higher humidity of 80% RH, it never shows a prolonged negative displacement. The positive displacement indicates that the formation of loose debris at the higher humidity does not play a significant role in separating the surfaces, since it only stay at the contact area for a short time. Small fluctuation of frictional force was observed at all humidity levels (Figs. 14(b) and 15(b)). There was no significant difference in the hardness of the adherent wear debris produced at all humidity levels. The hardness of the debris adhering on the

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Fig. 11. Scanning electron micrographs of the worn surface of aluminium pin after sliding at 28% RH (a) and 80% RH (b). At 28% RH, wear debris piled up on the worn surface formed a large pit.

Fig. 12. Scanning electron micrograph of the aluminium debris layers on pile up on the worn surface of the pin after sliding at 28% RH.

Fig. 13. Optical micrograph of the worn surface of aluminium pin after sliding at 28% RH revealed three main regions, (A) grooved metallic looking region, (B) bright metallic raised plateau and (C) debris layer.

Fig. 14. Variation of the displacement (a) and frictional force (b) during sliding of aluminium at 28% RH. In the initial distance of 70 m, the pin was lifted up above the starting contact point.

Fig. 15. Variation of the displacement (a) and frictional force (b) during sliding of aluminium at 80% RH.

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Fig. 16. Hardness profile of aluminium pin after sliding at 28% RH. ‘A’ and ‘B’ are the hardness values of the debris adhering on the worn surface. Other hardness values are obtained on the subsurface of the pin.

worn surface of the pin was found to be between 110 and 130 HV (Fig. 16).

between the intimate surfaces producing transfer elements of either surface portion and adhered onto the mating surface. Such transfer element is smaller than the typical debris particles. The repetition of accumulation of transfer elements leads to the formation of transfer particle which gradually grows to a considerable size. The transfer particle is compressed by the contact load and sheared. These flattened transfer particles are accumulated and piled-up on the surface before being removed as wear particles. This reflects that oneway, mutual or back transfer may occur depending not only on the mating materials but also on the tendency of the material for adhesion. Transfer material adheres not only to the pin surface but also to the disc surface. At lower humidity, it is possible that due to lack of moisture being adsorbed on the contact surfaces, the transfer particles have higher tendency to adhere to the pin and back transfer of loose debris from the disc to the worn surface of the pin is likely to take place. The debris particles adhered onto the pin separate the contact surfaces and thus reduce the interaction of the nascent surfaces.

3.4. Aluminium—discussion

4. Conclusions

The presence of the water vapour within the test enclosure undoubtedly affected the wear behaviour of aluminium. Water vapour easily adsorbs on both the freshly created surface and the wear debris generated, so that the debris egresses from the contact area without adhering to the parent surface. It has been reported that in fretting wear of aluminium, moisture in air adsorbed on the worn surfaces and the debris can cause the debris to egress from the contact area, exposing the nascent surface to oxidation wear [10]. Oxidation wear of aluminium alloys can take place at elevated temperature to produce oxide-rich debris [20]. The hardness of the debris adhering on the worn surface of the pin was found to be less than 150 HV. This result shows that the wear debris was not rich in oxide (as the hardness of aluminium oxide is higher than 1000 HV) and therefore oxidation wear is not likely the dominant wear mechanism. Free rolling and sliding of loose particles caused soft abrasion and thus grooves to form. High stress acting on the wear debris caused fracture to take place and thus the formation of a striation pattern running approximately perpendicular to the sliding direction. Moisture can promote the formation of debris by propagation of cracks at the subsurface as moisture accelerates the rate of fatigue crack growth in aluminium alloys [8]. The pins tested at all humidity levels were sectioned and examined, but no crack was observed at the subsurface. This result shows that the dominant wear mechanism of the pin is not associated with subsurface cracking and therefore the increase in the wear with humidity is not be due to an increase in the degree of cracking. The mechanism for the production of wear particles can be associated with the metal to metal interaction case [21]. This involves the shearing of metal–metal junction formed

The sliding wear of steel showed an increase of nearly 1.5 orders of magnitude when the relative humidity of the surrounding air decreased from 80 to 28% RH. At low humidity levels, both delamination and adhesion wear occurred. Delamination wear produced flake-like debris. The rupture of the existing adhesive bridges resulted in the liberation of small debris and thus the formation of fine cavities on the worn surface of the pin. It can be concluded that the mild wear occurring at high humidity levels is due to the inhibition of these wear mechanisms by the formation of interfacial layers such as iron hydroxide and ferri-oxide-hydrates, and the adsorption of water on the worn surface in addition to the normal atmospheric oxidation. Increasing the humidity from 28 to 80% RH increased the wear rate of aluminium by nearly half an order of magnitude. It is proposed that at higher humidity levels, water vapour adsorbs on both the freshly created surface and wear debris generated and therefore the wear debris egresses easily from the contact area without adhering to the parent surfaces. The debris particles adhered onto the pin played a significant role in separating the contact surfaces and thus reduced the interaction of the worn surfaces. The mechanism for the production of wear particles can be associated with the metal to metal interaction case. Oxidation wear and surface fracture are not likely to have played a significant role in the sliding wear of aluminium. References [1] D. Klaffke, On the repeatability of friction and wear results and on the influence of humidity in oscillating sliding tests of steel–steel pairings, Wear 189 (1995) 117–121.

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