磷酸盐7

Oxidation Resistance and High-Temperature Lubricating Properties of Magnesium-Phosphate-Treated Graphite Hideki Kita,w Manabu Fukushima, and D. Doni Jayaseelan National Institute of Advanced Industrial Science and Technology (AIST), Nagoya 463-8560, Japan Yu-Ping Zeng Shanghai Institute of Ceramics, Shanghai 200050, China Kazuo Ohsumi Isuzu Advanced Engineering Center Ltd., Fujisawa-shi 252-8501, Japan The oxidation resistance and lubricating properties of graphite treated with magnesium phosphate were investigated. Magnesi- um-phosphate-treated graphite showed improved oxidation re- sistance. That is, with magnesium-phosphate treatment, the oxidation temperature of graphite increased by almost 2001C. Furthermore, the friction coefficient of magnesium-phosphate- treated graphite measured at 5001C was about 0.05, which was smaller than that measured at room temperature. I. Introduction GRAPHITE, which has a unique stack structure that enables asmall friction coefficient, is used widely as a solid lubri- cant.1–8 However, the oxidation of graphite remains a great concern for using it at high temperatures in air. For this reason, its use is limited to a low-temperature range. In order to make use of its solid lubricant property, its oxidation resistance prop- erty should be improved and its friction coefficient should be reduced. Many techniques have been used to improve the oxi- dation resistance of graphite and bulk carbon materials, such as oxide and metal particle addition and glass coating on graphite surface to prevent oxidation.9–13 For example, graphite powder is sprayed on mold surfaces to reduce frictional resistance and prevent sticking in the metal-forging process. Graphite materials with excellent oxidation resistance will not only improve processing efficiency, but also extend the service life of molds and modify the quality of products. However, if graphite is used as a solid lubricant for sliding contact, it can be easily oxidized because of the damage of coating layer. Furthermore, the addi- tives may separate from graphite particles, resulting in the in- crease in the friction coefficient. It was reported that magnesium phosphate could improve graphite oxidation resistance.14,15 Nevertheless, the mechanism remains unclear and its effect on friction properties has not been reported yet. Hence, in this study, the oxidation resistance and high-temperature lubricating properties of graphite treated with magnesium phosphate were investigated, and the feasibility of graphite as a solid lubricant at high temperature was also discussed. II. Experimental Procedures (1) Specimen Preparation Table I shows the properties of the commercial graphite powder used in the present study. First, Mg (H2PO4)2 �H2Owas dissolved in distilled water to prepare aqueous solutions of 1, 5, 10, 20, and 30 wt% concentration and 30 wt% graphite powders were then added to Mg(H2PO4)2 �H2O solutions. The graphite slurries con- taining different Mg(H2PO4)2 �H2O concentrations were ball mixed for nearly 12 h and oven dried at 1101C for 24 h. The dried powders were crushed, sieved to pass through a 100 mm mesh size, and calcined at 8001C for 3 h in nitrogen atmosphere to get magnesium-phosphate-treated/coated graphite particles. (2) Characterization Thermogravimetric analysis (TGA, TAS300, Rigaku Ltd., To- kyo, Japan) was employed to measure the weight loss of MgP2- coated graphite powders. The heat rate is 101C/min up to 10001C. The sample was put in a platinum pan and was heat- ed in natural air atmosphere. Phase analysis was carried out by X-ray diffraction using CuKa as radiation (MXP 18, Mac Sci- ence Co. Ltd., Tokyo, Japan). The friction coefficient was meas- ured using the pin-on-disc method; the tester is shown in Fig. 1. A silicon nitride pin of diameter 5 mm � length 15 mm and a silicon nitride disc of diameter 50 mm � thickness 5 mm were used in the present study. Tests were conducted at both 201 and 5001C for above 30 min at measuring point. The load and slid- ing velocity in the experiment were 9.8 N and 2.5 m/s, respec- tively. As a solid lubricant, graphite powder was placed on the plate so that, it infiltrated into the gap between the pin and the plate followed by the rotation of the plate. III. Results and Discussion Thermogravimetry-differential thermal analysis (TG-DTA) was carried out for both Mg(H2PO4)2 �H2O- and Mg(H2PO4)2 � H2O-treated graphite particles. The TGA of Mg(H2PO4)2 �H2O (not shown here) showed two weight loss stages at 1301 and 2201C. Correspondingly, two exothermic peaks were observed at 1301 and 2201C. The exothermic peak at 1301C was respon- sible for the release of H2O, whereas dehydration and crystal- lization of Mg(PO3)2 from Mg(H2PO4)2 �H2O were responsible for the exothermic peak at 2201C. The X-ray diffraction patterns indicated that the composition of magnesium phosphate was strongly dependent on calcinations temperature. Likewise, mag- nesium-phosphate-coated graphite powders were complex in phase until it was fired up to 8001C and there were other dif- ferent phases present in the system. However, the powders cal- Journal J. Am. Ceram. Soc., 88 [9] 2632–2634 (2005) DOI: 10.1111/j.1551-2916.2005.00472.x r 2005 The American Ceramic Society 2632 N. S. Jacobson—contributing editor wAuthor to whom correspondence should be addressed. e-mail: hideki-kita@aist.go.jp Manuscript No. 20218. Received December 15, 2004; approved February 25, 2005. cined at 8001C for 3 h showed well-defined peaks of graphite and Mg(PO3)2 (or MgH2P2O7). Figure 1 shows thermal analysis curves of graphite with and without magnesium-phosphate treatment. The results showed that weight loss starting temperature (Ts) and completion tem- perature (Tc) are closely associated with concentration of mag- nesium phosphate. Ts and Tc of untreated graphite were about 6001 and 8001C, respectively. After magnesium-phosphate treat- ment, both Ts and Tc increased further. For example, Ts and Tc of 30 wt% solution-treated graphite were 7501 and 9501C, re- spectively. It was inferred from this simple TGA that oxidation resistance was improved by magnesium-phosphate treatment. Figure 2 shows friction coefficients of graphite measured at different temperatures. The friction coefficient of untreated graphite was about 0.14 as reported previously. With magnesi- um-phosphate addition, the friction coefficient decreased initial- ly and then increased. The friction coefficient of graphite treated with 30 wt% magnesium phosphate at 201C was about 0.2. The friction coefficient of magnesium-phosphate-treated graphite measured at 5001C was much smaller (0.05) than magnesium- phosphate-treated graphite measured at 201C, and it was almost same irrespective of magnesium-phosphate concentration. That is, with the increase in magnesium-phosphate concentration, the variation of the friction coefficients was not serious. The friction coefficient of untreated graphite could not be measured at 5001C because of the oxidation problem. The above results revealed that, magnesium phosphate coat- ed on the surface of the graphite particles resulted in an increase in friction coefficient, and magnesium-phosphate coating layer prevented graphite from high-temperature oxidation. During the test, the solid–solid sliding would break the magnesium- phosphate coating layer. Therefore, the friction coefficient in- creases with the increase in magnesium-phosphate concentra- tion. At 5001C, the self-lubricant properties of graphite make its friction coefficient slightly increase, resulting in an overall low friction coefficient. Figure 3 shows a schematic drawing of the atomic structure of graphite. It is supposed that the endmost carbon atoms in basal planes of a graphite molecule are active because of unoc- cupied electrons. Therefore, they readily combine with an oxy- gen atom, resulting in oxidation. It may be assumed that a Mg(PO3)2 molecule bonds with the endmost carbon atom of graphite stacks and makes it more stabilized; therefore, the ox- idation resistance improves and the attractive force between the two stacks is reduced, which results in the decrease in the friction coefficient. However, the detailed mechanism is a future subject, which remains to be clarified. IV. Conclusions Oxidation resistance and high-temperature lubricating proper- ties of graphite treated with magnesium phosphate were inves- tigated. The oxidation temperature increased with the increase in the concentration of magnesium phosphate. The sliding tests showed that the magnesium-phosphate-treated graphite had very small and stable friction coefficients at 5001C. References 1F. P. Bowden and D. Tabor, The Friction and Lubrication of Solids, Part II, p. 187. Oxford University Press, London, 1964. 2R. H. Savage, ‘‘Graphite Lubrication,’’ J. Appl. Phys., 19, 1–10 (1948). 3J. K. Lancaster, ‘‘Transition in the Friction and Wear of Carbons and Graph- ites Sliding Against Themselves,’’ ASME Trans., 18, 187–201 (1975). 4J. K. Lancaster and J. R. Pritchard, ‘‘The Influence of Environment and Pres- sure on the Transition to Dusting Wear of Graphite,’’ J. Phys. D: Appl. Phys., 14, 747–62 (1981). 5R. F. Deacon and J. F. Goodman, ‘‘Lubrication by Lamella Solids,’’ Proc. R. Soc. Lond., A243, 464–81 (1958). 6R. A. Pallini and L. D. Wedeven, ‘‘Traction Characteristics of Graphite Lu- bricants at High Temperature,’’ Tribol. 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