Oxidation Characteristics of Molybdenum Disulphide

The specific surface area of the molybdenum disulphide particles affects the rate of oxidation . Finely divided powders have a high specific surface, and tend to be more rapidly oxidised, but larger particles are not necessarily resistant to oxidation, since they may consist of porous agglomerations of fine particles. Ducas found that a sample ground to give a powder with a specific surface of 30 m /g gave sulphuric acid in 10% yield when extracted with water after exposure to air of 80% relative humidity for one month. The actual particle size was not stated, but a specific surface of 30 m /g represents a particle size of the order of 0.1//m. A more representative powder for lubricant use would have a particle size of about 1-3//m, and oxidation of such a powder would be far slower, probably far less than 1% oxidation under the same period and conditions of exposure. 

While studying the influence of molybdenum disulphide on the wettability behaviour of steel, Braithwaite and Greene found that the pH of a powder fell from 6.07 to 3.55 in one hour and to 2.60 in two hours when heated at 350°C in air. This powder was more typical of a fine lubricant grade, having a BET specific surface of 3 m /g, but the temperature used was very much higher and it is difficult to compare their results with those of Ducas because of the different technique used to assess the acidity produced. 

Braithwaite and Greene also gave some figures for the pH of some powders of different particle size, and these are shown in Table 4.3. The acidities of these powders are probably due to the grinding process and to subsequent atmospheric oxidation, but they confirm the general tendency for a higher level of oxidation and acidity with finer powders. 

The normal stable product of oxidation is molybdic oxide or molybdenum trioxide MoOj, and this oxide is not abrasive, so that satisfactory lubrication can be obtained with molybdenum disulphide even after considerable oxidation. The cohesion of molybdic oxide and its adhesion to metal surfaces are inferior, however, and ultimately films will fail for these reasons. The presence of the trioxide causes 

Molybdenum dioxide is highly abrasive, and its formation would be a serious disadvantage to lubrication performance, but in practice it seems clear that the formation of the dioxide is transitory or exceptional, and that the oxidation process normally produces the more harmless trioxide. 

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Figure 4.4 Oxidation Characteristics of Molybdenum Disulphide (Refs.89-93)...

Figure 4.4 summarises the oxidation characteristics of molybdenum disulphide from room temperature to 610°C, the oxidation rates being compared with the rate at 490°C. It is clear that no arbitrary temperature can be defined below which oxidation does not occur, but that the oxidation rate is extremely small below 400°C, and Sliney found that the rate constant at 370°C was only 6 x 10 for surface-layer oxidation and 2 x 10 for bulk oxidation. 

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