| The output of rice husks is extremely huge every year, while rice husks are always be disposed as agricultural wastes, resulting in a waste of resource and environmental pollution.Silica is rich in rice husks, and Si element also plays a very important role in high-performance titanium materials. Therefore, it is meaningful to take full advantage of the abandoned rice husks, and prepare the SiO2/Ti composites by using rice husk silica. In the present paper, rice husks were pickling-pretreated by citric acid solution. The thermolysis process of rice husks and kinetic parameters were identified by Differential scanning calorimetry ?DSC? combined with kinetic calculation. The composition of pyrolysis residue were analyzed by carbon-sulfur analyzer,XRD and XPS analysis. Then SiO2/Ti and Si/Ti, TiO2/Ti composites with the same Si or O were prepared by hot-press sintering. The lattice distortion and hardness of composites caused by the content of Si and O were investigated, and strengthening mechanism of SiO2/Ti composite was studied. Finally, relationships between the content of rice husk silica and the densification, microstructure,hardness friction and wear performance of composites were studied. The main conclusions are presented as follows.1. The thermolysis process of rice husks can be divided into five stages: loss of free water?below 80??,loss of crystal water ?80?250??,thermolysis and carbonization ?250?360??,combustion of carbon ?360?600??, and stage of stabilization ?above 600??. During 250?360?, the rate of weight loss is 46%, the speed of weight loss is the fastest, and the corresponded activation energy of reaction ?77.13kJ·mol-1? is also the largest. After acid-pickling treatment and fully burning in air at 900?, the component of pyrolysis residue is high-purification amorphous silica, the carbon content falls to only 0.17wt.%,. The average particle size of pyrolysis residue is 10.42?m after milling for 4 hours.2. When the content of SiO2 is less than 1.00wt.%, the mainly existent forms of Si and O elements are solid solution in titanium matrix, and the SiO2/Ti composites nearly consist of Ti solution grains, while only a very little sillica was remained on the primary powder boundary?PPB?. When the content of SiO2 is 2.00wt.%, a small amount of a new TiSi2 phase appears.3. Within the solid solubility of Si and O elements, the interstitial solid solution of O leadsto the expansion of a-Ti crystal lattices. The relationships between the lattice constant and O content can be expressed as: c[O]wt% 4.684 + 0.046·O%,a[O]wt% 2.953 + 0.014·O% . The substitiutional solid solution of Si leads to the contraction of a-Ti crystal lattices. And relationships between the amount of contraction and Si content can also be expressed as:?c[Si]wt%, = -0.050 · Si%, ?a(Si]wt% ?0.020 · Si% ?Si%?0.468wt.%? ?4. The strengthening effect caused by the substitutional solid solution of Si element is obviously weaker than that of interstitial solid solution caused by the interstitial solid solution of O element. The hardness ?HV? of titanium materials strengthened by Si and O solutions can be expressed as : HV= 178.86+209.43·O%+98.50· Si% ?Si%?0.468wt.%?.5. Compared with pure titanium, the wear resistance of SiO2/Ti composites enhances obviously. Within the solid solubility of Si and O elements, the wear rate decreases gradually with the increase of SiO2 content. Under the condition of dry friction and wear, the main wear mechanism of pure titanium is adhesive wear, while that of SiO2/Ti composites is abrasive and adhesive wear. |
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