B4C/hBN复相陶瓷的摩擦化学行为

Boron carbide (B4C) has many outstanding properties, such as high hardness, good chemical stability and high neutron absorption cross section, and could be used as friction pairs with ceramics or metals in many situations. However, its engineering application has been restricted mainly due to its poor machinability and tribological performance. Adding second phases into ceramic matrixes is an effective way of improving the performance of ceramics, and is becoming one of hotspots in the research of ceramic materials. Recent studies have shown that the machinability of B4C ceramics can be improved obviously while hexagonal boron nitride (hBN) added into its matrix. Moreover, it was found that hBN has an important influence on the tribological performances of Si3N4 and Al2O3 ceramics. However, the research on the tribology of B4C/hBN composites is still blank up till now. The object of present work is to research systematically the tribological characteristics of B4C/hBN composites, in which the key point is their tribochemical behavior. The main contents of the work are: (1) tribological properties of B4C/hBN composites at different frictional conditions; (2) constitutions and properties of tribofilms formed on the wearing surfaces; (3) the effect of interface between B4C and hBN on the film formation; (4) the mechanisms of the film formation and its lubrication. The main characteristic of the work is to research the interface of B4C/hBN composites combining test research with simulative calculation. The purpose of the work is to grasp the tribological law of B4C/hBN composites and to reveal the tribochemical mechanisms of the composites.

B4C陶瓷具有高硬度、优异的化学稳定性和高的中子吸收截面等突出特点，在很多场合下可与陶瓷或金属构成摩擦副。然而由于其可加工性能和摩擦学性能较差，从而限制了其工程化应用。陶瓷基体中引入第二相，是改善其性能的有效途径，并成为当今陶瓷材料研究热点之一。近期已有研究表明，将hBN引入B4C陶瓷中可明显改善其加工性能，并发现hBN对Si3N4、Al2O3等陶瓷材料的摩擦学性能影响显著，但目前关于B4C/hBN复相陶瓷的摩擦学研究尚属空白。本项目拟系统研究B4C/hBN复相陶瓷的摩擦学特性，重点研究其摩擦化学行为。主要内容有：①不同摩擦条件下的摩擦学性能表征；②摩擦面上形成的表面膜组成与性能分析；③复相陶瓷的界面对表面膜形成的影响；④探索表面膜的形成机理与润滑机制。复相陶瓷界面的试验研究与模拟计算相结合是本项目主要特色；掌握B4C/hBN复相陶瓷的摩擦学规律，揭示其摩擦化学机理，是本项目的主要目的。

碳化硼(B4C)陶瓷具有高硬度、低密度、优异的化学稳定性等优良特点，在工程领域应用广泛。但由于其与陶瓷、金属配副的摩擦学性能较差，制约了其在耐磨材料领域的应用。将固体润滑剂加入陶瓷基体中构成自润滑陶瓷可有效改善陶瓷的摩擦学性能。本项目将六方氮化硼(hBN)引入B4C基体中，通过热压烧结的方法制备B4C/hBN复相陶瓷，系统研究了B4C/hBN与不同材料配副的摩擦学性能。本项目的创新性成果主要体现在：1)干摩擦条件下，B4C/hBN与纯B4C配副时，配副方式对摩擦学性能影响显著。当采用上销下盘的配副方式时，摩擦过程中产生的摩擦产物驻留在盘表面，构成磨粒磨损，此时hBN的加入无法改善复相陶瓷与纯B4C配副的摩擦学性能；当采用上盘下销的配副方式下，大部分磨屑由于重力作用脱离摩擦表面，摩擦过程中在盘的摩擦表面形成了含硼酸(H3BO3)的表面膜，磨损机制由磨粒磨损转为化学磨损，大幅改善了摩擦学性能。2)蒸馏水润滑条件下，B4C/hBN与纯B4C配副时，复相陶瓷中的hBN可与水分子发生反应生成H3BO3，从而在摩擦表面形成润滑膜，大幅降低了配副的摩擦因数。3)干摩擦条件下，B4C/hBN与奥氏体不锈钢(ASS)配副时，可在销、盘试样表面同时形成含B2O3和Fe2O3的氧化膜，摩擦因数可由纯B4C与ASS配副的0.631降低至BN30与ASS配副的0.081。4)干摩擦条件下，B4C/hBN与灰铸铁配副时，试验载荷对摩擦学性能影响重大，当载荷为5N时，BN30-HT摩擦副的销表面可形成含B2O3和Fe2O3的表面膜，摩擦因数由BN00-HT时的0.5降低至BN30-HT时的0.2，当载荷增至10N时，BN30销表面膜碎裂，磨损机制由5N载荷时的化学磨损转为10N载荷下的磨粒磨损，摩擦因数升至0.6。5)乳化液润滑条件下，B4C/hBN与灰铸铁配副时，当采用浸入法乳化液润滑时，所得摩擦产物被冲入乳化液中，构成磨料磨损，当采用滴定法乳化液时，所得摩擦产物可覆盖在复相陶瓷销表面，构成化学磨损，摩擦因数降低至0.008。6)采用密度泛函理论(DFT)进行了体相及表面计算，获得了B4C和hBN的基态性质、表面能等参数，为探索B4C/hBN复相陶瓷的润滑机制提供重要理论指导。