| The grease is indispensability lubricant for the mechanical equipment to reduce energy consumption and prolong the life of the machine. Preparation of biodegradable grease is a topic at the forefront of world concern, and titanium complex grease is also a kind of biodegradable grease. In order to accelerate industrial application of titanium complex grease in china, different titanium complex greases were synthesized in laboratory, i.e., based oil consisted of 650SN, or neopentyl polyol ester, or mixed oil (neopentyl polyol ester: 650SN: epoxidized soybean, 4.5:2.5:1, or neopentyl polyol ester: 650SN, 1.8:1), and compositions consisted of benzoic acid/stearic acid or sebacic acid/stearic acid, and additives consisted of PTFE, or TiO2 nanoparticles, or SiO2 nanoparticles.Dropping point and penetration of titanium complex grease were measured according to national standards GB/T 3498 and GB/T 269, similar to ASTM D2265 and ASTM D217. The tribological behaviors of titanium complex greases were characterized by a four-ball tester. The worn surface was investigated with the use of scanning electron microscope and X-ray photoelectron spectrometer. The structure of titanium complex soap molecule after degradation and the titanium complex soap molecule were characterized with the use of FTIR spectrophotometer. The effect of different based oils, different compositions and different additives on physical characteristics, tribological properties and biodegradability of titanium complex greases were analyzed, and that the mechanism of lubrication and biodegradation of titanium complex greases were studied. At the same time, the differences of the tribological properties between titanium complex grease and diurea grease and lithium grease were studied.Titanium complex greases were prepared under the following procedures. After which additional of organic acid, titanium isopropoxide were added. Saponified temperature, saponified duration, synthetical velocity, refined temperature, and refined duration had an effect on physical characteristics and tribological properties of titanium complex greases. FTIR results of titanium complex greases disclose that soap molecule structures of benzoic acid/stearic acid and sebacic acid/ stearic acid titanium complex grease were synthesized by the above-mentioned synthetic process. When the consistency concentration was 17%, synthetical velocity of neopentyl polyol ester-based titanium complex grease was quicker than that of other oils-based titanium complex greases, and that reactive degree of benzoic acid/stearic acid and sebacic acid/stearic acid titanium complex grease was higher and above 80% as compared with that of acetic acid/stearic acid and salicylic acid/stearic acid titanium complex greases. At the same time, different additives at concentration of 0.5–3.0 wt% remarkably had no effect on the dropping point of different titanium complex greases, but had an effect on the penetration of different titanium complex greases. Appearance of different titanium complex greases varied from buff to brown. Furthermore, the consistency number of mixed oil-based or 650SN-based titanium complex greases was No. 2 (NLGI 2 hardness), and that the corresponding dropping point was above 280°C. However, the consistency number of neopentyl polyol ester-based titanium complex grease was No. 4 (NLGI 4 hardness), and that the corresponding dropping point was below 200°C. Moreover, the dropping point of sebacic acid/stearic acid titanium complex grease was higher than that of benzoic acid/stearic acid titanium complex grease.For friction-reduction and antiwear properties of different titanium complex greases, friction-reduction properties of 650SN-based benzoic acid/stearic acid titanium complex grease were better than those of other types of oil-based titanium complex grease, and that the antiwear properties of mixed oil-based titanium complex grease containing epoxidized soybean were better than those of other types of oil-based titanium complex grease. Moreover, compositions had no effect on the tribological properties of different titanium complex greases for the same oil-based greases. To some extent, the friction coefficients and the wear scar diameters lubricated by titanium complex grease containing PTFE, TiO2 nanoparticles, or SiO2 nanoparticles were reduced, and that the optimum concentrations of PTFE, TiO2 nanoparticles, or SiO2 nanoparticles are approximately 2.0, 0.5, and 1.5 wt%, respectively. Furthermore, for mixed oil-based titanium complex greases (neopentyl polyol ester: 650SN: epoxidized soybean, 4.5:2.5:1), the friction system could be lubricated effectively to load of 200–500 N by adding PTFE, TiO2 nanoparticles, or SiO2 nanoparticles. At the same time, the load-carrying capacity of mixed oil-based titanium complex grease was increased by 37.5% as compared with that of other types of oil-based titanium complex greases, i.e., the PB value of mixed oil-based titanium complex grease was 1100 N. Compositions had no effect on the load-carrying capacity of different titanium complex greases for the same oil-based greases. Compared with diurea grease and lithium grease, the tribological performance of all titanium complex grease was better than that of lithium grease and diurea grease.The main wear mechanism of steel balls lubricated by different titanium complex greases shows slight adhesive wear. The reason that adhesive wear slowed down is that a synergistic effect between the adsorptive film and titanic chemical sediment on the steel balls worn surface. A synergistic effect between the adsorptive film formed by titanium complex grease containing different additives and the chemical sediment of titanium dioxide during friction process contributes to improving tribological properties. Moreover, the boundary film on the worn surface was mainly composed of titanium complex grease itself, different additives (PTFE, TiO2 nanoparticles, or SiO2 nanoparticles), and titanic chemical sediment.Results of the primary biodegradability show that the biodegradation level of sebacic acid/stearic acid titanium complex greases was significantly higher than that of benzoic acid/stearic acid titanium complex greases with 21 days, and that of neopentyl polyol ester-based titanium complex grease was above 65%, and that biodegradation percentages ranged between 65% and 83%. Moreover, the biodegradation level of sebacic acid/stearic acid titanium complex grease seemed to be independent of (2.0 wt%) different additives which may affect biodegradability, and that of benzoic acid/stearic acid titanium complex grease was determined by different additives and benzene ring structure of the grease itself which may affect biodegradability. The results also indicate that the titanium complex soap molecule was degraded by means of hydrolysis of ester bonds,β-oxidation of long chain hydrocarbon, or oxidation degradation of benzene ring.
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