FRICTION AND WEAR OF MOVING SURFACES IN ABRASIVE CONTAMINATED ENVIRONMENT

FRICTION AND WEAR OF MOVING SURFACES IN ABRASIVE

CONTAMINATED ENVIRONMENT

Ezzat A. A.* and Mousa M. O.**

*El-Minia High Institute of Technology, El-Minia, EGYPT.

**Faculty of Engineering, Minia University, P. N. 61111, El-Minia, EGYPT.

ABSTRACT

In the present work, graphite (C) and polymethyl  methacrylate (PMMA) were used as solid lubricants dispersed in lithium grease. Their effect on friction and wear of moving surfaces contaminated by solid contaminants in the cement plant is discussed. The tests were carried out at sliding velocity of 0.5 m/s and load of 10 N. The rotating specimens were greased before the test and further greasing was carried out every 30 second during the test. The test time was 5 minutes. The wear scar diameter was measured for the upper stationary pin by using an optical microscope with an accuracy of ± 1.0 µm. Experiments were carried out at 25 °C. Based on the experimental results , it was found that, graphite caused slight increase in friction and significant reduction of wear, while PMMA in most case causes decrease in friction and wear. Besides, wear and friction decreased with increasing oil content in grease, Wear and friction decreased with increasing PMMA. Graphite displayed low wear and The friction coefficient increases gradually when  graphite content  increased.

KEYWORDS

Friction, wear, grease, solid contaminants, cements plant, graphite, polymethyl methacrylate.

INTRODUCTION

The behaviour of many materials is closely related to their hardness, [1]. However, classical wear models do not take into consideration the contact scale dependence of hardness, an effect commonly known as indentation size effect (ISE). Scores of abrasive wear problems particularly those in the harsh environments involve remarkable breakage of the abrasive, [2]. In desert areas, abrasive particles entering the machines cause serious wear of the sliding components, [3, 4]. Abrasive wear of composite materials is a complicated surface damage process, affected by a number of factors, such as microstructure, mechanical properties of the target material and the abrasive, loading condition, environmental influence, etc. Microstructure is one of the major factors; however, its effect on the wear mechanism is difficult to investigate experimentally, [5, 6], due to the possible synergism with other influences. Lubrication is critical for minimizing wear in mechanical systems, [7], that operate for extended time periods. Developing lubricants that can be used in engineering systems without replenishment – particularly those that are environmentally friendly – is very important for increasing the functional lifetime of mechanical components. White Portland cement or white ordinary Portland cement (WOPC) is similar to ordinary, gray Portland cement in all respects except for its high degree of whiteness, [8]. The raw materials involved in white cement are Sand (80 %), limestone (12 %) and kaolin(8 %).

Interest has risen in solid powder lubrication due to its proven ability to provide low friction and wear in interfaces unsuitable for traditional oils. This may be in the form of augmenting oil performance as an additive, or in the form of thin, solid transfer films since it was found that sliding materials sometimes inherently generate a film that can protect the contact interface during relative motion, [9 -10]. Graphite is used as lubricant in machines which have to be operated at high temperatures. All such machines cannot be lubricated with oils, grease, etc. As they vaporize immediately at the high temperature. As a lubricant it is used as dry powder or mixed with water or oil, [11]. Poly (methyl methacrylate) or PMMA is the most commonly used polymer among the methacrylate family and has found tremendous application in automotive and home appliances. PMMA is one of the most polymers commonly used in the plasticized polymer electrolytes. PMMA, [12], with another  polymer that can provide a good mechanical property.

In the present work, graphite (C) and polymethyl methacrylate (PMMA) were used as solid lubricants dispersed in lithium grease. Their effect on friction and wear of moving surfaces contaminated by solid contaminants in the cement plant is discussed.

EXPERIMENTAL WORK

Tests were carried out at sliding velocity of 0.5 m/s and load of 10 N. The rotating specimens were greased before the test and further greasing was carried out every 30 sec during the test. The test time was 5 min. The wear scar diameter was measured for the upper stationary pin by an optical microscope with in an accuracy of ± 1.0 µm. Experiments were carried out at 25 °C using lithium based grease. The solid lubricant additives were PMMA and graphite.Experiments were carried out using a cross pin tester, Fig 1. It consists, mainly, of rotating and stationary pins of 18 mm diameter and 180 mm long. The rotating pin was attached to a chuck mounted on the main shaft of the test rig. The stationary pin was fixed to the loading block where the load is applied.The main shaft of test machine is driven by DC motor (300 watt, 250 volt) through reduction unit. Moreover, the motor speed is adjustable and can be controlled by varying the input voltage using an auto transformer.  The test rig is fitted by a load cell to measure the frictional force generated in the contact zone between the rotating and stationary pins. Normal load was applied by means of weights applied to a loading lever. A digital screen was connected to the load cell to detect the friction force. Friction coefficient was determined by the ratio between the friction force and normal load and wear was determined by measuring the wear scar diameter by the optical microscope.

Shell Alvania Greases EP (LF), [13], were used in the experiments. They are multipurpose, extreme - pressure industrial greases based on a blend of high viscosity index mineral oils a lithium hydroxystreate soap thickener and contain lead free extreme – pressure and other additives.

Fig. 1  Layout of the test rig.

RESULTS AND DISCUSSION

The relationship between wear and oil content in lithium grease is shown in Fig. 2. As seen, wear decreased with increasing oil content, where wear scar diameters were 2.0 and 1.5 mm at 0 and 30 % oil content respectively. This behavior means that when the oil content increased the wear decreased. As the oil content increased the grease was well distributed over the contact area and grease film was formed leading to wear decrease.

The effect of sand content, in lithium grease diluted by different oil content, on wear is shown in Fig. 3. When the sand content increased wear increased because sand is abrasive material and it is well known that the hardness of sand is higher than steel, so the sharp edges of sand particles abraded the steel surface. It can be seen that at 25 % sand content 30 % oil is the best but at 15% sand the 10% oil in lithium grease is enough to reduce the wear scar diameter. When oil content increased wear scar diameter decreased.

The effect of kaolin content on wear in lithium grease diluted by different oil content is shown in Fig. 4. As kaolin content increased wear increased because kaolin is abrasive material and the hardness of kaolin is higher than steel, so that the sharp edges of kaolin particles abraded steel. Besides, wear decreased with increasing oil content, where wear values were 2.4 and 1.5 mm at 0 and 30% oil content respectively. This behavior means that when the oil content increased the wear decreased.

The relationship between wear and limestone contaminating lithium grease containing different oil content is shown in Fig. 5. As limestone content increased wear increased. Wear decreased with increasing oil content, where wear values were 2.0 and 1.6 mm at 0 and 30% oil content respectively.

The effect of the mixture of sand, kaolin and limestone contaminating lithium grease with different oil content is illustrated in Fig. 6. The wear increase is attributed to the fact that the mixture of the contaminants containinga highpercentage ofsandup to 80%, and as thesandincreased thewearincreasedbecause the effect ofthe sand in the mixtureispredominantly.

The effect of oil content  in lithium grease on the friction coefficient is shown in Fig. 7. Friction decreased with increasing oil content, where friction values were 0.025 and 0.02 at 0 and 30% oil content respectively.

Figure 8 shows the relationship between friction and sand content in lithium grease with different oil content. When sand content increased friction increased, where friction coefficient values were 0.075 and 0.05 at 0 and 30% oil content respectively. It seems that increasing oil content decreased friction coefficient. This behavior shows that when the oil content increased the friction decreased, As the oil content increased the grease  was well distributed over the contact area and grease film was formed  and consequently friction decreased.

Figure 9 shows the relation between friction and kaolin content in lithium grease with different oil content. As seen in the figure that when the kaolin content increased the friction increased because kaolin is an abrasive material, where friction values were 0.09 and 0.07 at 0 and 30% oil content respectively.

The same trend was observed for limestone in lithium grease with different oil content, Fig. 10. Friction values were 0.03 and 0.02 at 0 and 30% oil content respectively. It seems that increasing oil content decreased friction coefficient between limestone particles and steel surfaces.

Figure 11 shows the relationship between friction and the contaminant mixture content in lithium grease with different oil content. When the mixture content increased the wear increased as the mixture consists of some of abrasive materials (sand, kaolin and limestone). Friction values were 0.035 and 0.02 at 0 and 30% oil content respectively.

Figure 12 shows the relationship between wear and sand content in lithium grease with different oil content and 5 % PMMA. As sand content increased wear increased. Wear values were 1.6 and 1.4 mm at 10 and 30% oil content respectively. Wear values at 0 % PMMA were 2.2 and 1.7 mm at 10 and 30% oil content respectively. It can be noted that wear at 5 % PMMA is lower than at 0 % PMMA.

The relationship between wear and sand content in lithium grease with different oil content and 15 % PMMA is shown in Fig. 13. Wear values were 1.3 and 1.4 mm 10 and 30% oil content respectively. Wear values at 5 % PMMA were 1.6 and 1.4 mm at 10 and 30% oil content respectively. Wear at 15 % PMMA is lower than at 5% PMMA.

Figure 14 shows the relationship between wear and mixture content in lithium grease with different oil content and 5 % PMMA. Wear values were 1.4 and 1.3 mm at 10 and 30% oil content respectively. This behavior confirms that when oil content increased wear decreased, as the oil content increased the grease  was well distributed over the contact area and grease film was formed and wear decreased. Wear values at 0% PMMA were 1.6 and 1.4 mm at 10 and 30% oil content respectively. Wear at 5 % PMMA is lower than at 0 % PMMA.

Figure 15shows the relationship between wear and contaminant mixture content in lithium grease with different oil content and 15 % PMMA. As the mixture content increased the wear increased. Wear decreased with increasing oil content, where wear values were 1.4 and 1.2 mm at 10 and 30% oil content respectively. This behavior indicates that when oil content increased wear decreased. As the oil content increased the grease  was well distributed over the contact area and grease film was formed and wear decreased. Where wear values at 5 % PMMA were 1.4 and 1.3 mm at 10 and 30% oil content respectively. Wear at 15 % PMMA is lower than at% PMMA. This is dueto PMMA is a good lubricant.

Figure 16 shows the relationship between friction and sand content in lithium grease with different oil content and 15 % PMMA. As seen in the figure, when sand content increased friction increased. Friction coefficient values at 0 % PMMA were 0.06 and 0.05 at 10 and 30% oil content respectively. Friction coefficient at 15 % PMMA is lower than at 0 % PMMA. This is dueto the fact that PMMA is a good solid lubricant which separates friction surfacesand reduces frictionby increasing the rolling motion of sand particles between the rubbing surfaces and consequently reduces friction.

The relationship between wear and sand content in lithium grease with different oil content and 5 % graphite is shown in Fig. 17. As sand content increased wear increased. Besides, wear decreased with increasing oil content, where wear values were 2.0 and 1.8 mm at 10 and 30% oil content respectively. This behavior shows that when the oil content increased the wear decreased, where wear values at 0% graphite were 2.3 and 1.7 mm at 10 and 30% oil content respectively.

Figure 18 shows the effect of the contaminant mixture content in lithium grease with different oil content and 5 % graphite on wear. As the mixture content increased the wear increased. It is also seen that wear decreased with increasing oil content, where wear values were 1.55 and 1.5 mm at 10 and 30% oil content respectively. This behavior indicated that when the oil content increased the wear decreased. Wear values at 0% graphite were 1.6 and 1.4 mm at 10 and 30% oil content respectively. It can be noticed that wear at 5 % graphite was lower than at 0 % graphite as graphite caused low wear as it covered the sliding surfaces and protected the surfaces from wear. Contaminant mixturecontaineda highpercentage ofsandup to 80%, and as thesandincreased thewearsignificantly increased because the effect ofthe sandmixturewaspredominantly.

The relation between friction and sand content in lithium grease with different oil content and 10 % graphite is shown in Fig. 19. When the sand content increased the friction increased. Friction decreased with increasing oil content, where friction values were 0.24 and 0.2 at 10 and 30% oil content respectively. It seems that increasing oil content decreased friction coefficient between sand particles and steel surfaces. This behavior means that when the oil content increased the friction decreased.

CONCLUSIONS

1. The contaminants in cement plant contains up to 80% sand content. Sand increases wear and friction as sand is an abrasive material of hardness higher than steel.

2. Wear and friction decrease with increasing oil content in grease.

3. Wear and friction decrease with increasing PMMA. This effect may be due to being PMMA is a good lubricant which works on the separation of friction surfaces and consequently reduces friction and wear by facilitating the rolling of abrasive particles between the rubbing surfaces.

4. Graphite causes low wear as it covers the sliding surfaces so that it protects thesurfaces from wear. The friction coefficient increased gradually when graphite content  increased because graphite has a slight abrasive action. Increasing PMMA up to 15 % showed that the best effect in reducing friction and wear. The lowest wear and friction coefficient were obtained from greasecontaining 15 % PMMA and 30 % oil.

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