强流脉冲电子束辐照WC-Co硬质合金表面纳米碳粒子析出行为及摩擦学性能研究

With the rapid development of modern processing and manufacturing industries, there is an increasing demand for WC-Co hard alloy with good performance. The low surface hardness and poor wear resistance constitute the inadequate factors of current hard alloy tools made of WC grains larger than 0.3μm.The service properties of WC-Co hard alloy can be improved effectively by using high energetic beam surface modification, where does not exist the consideration of adhesion strength inherent in the coating technologies. However, the surface modification techniques with high energetic beams have been limited greatly due to the requirement of complicate and costly equipments. In the previous work, the surface modification of WC-Co hard alloy was carried out with a self-made high current pulsed electron beam (HCPEB) device designed by the applicant. The in-situ precipitation of dispersed nano carbon partilcles (NCPs) from WC grains was discovered in the surface modified layer, and this phenomenon brought a great improvement in the wear resistance of more than ten times.Up to now,the formation mechanism of NCPs and its effect on the wear property of WC-Co hard alloy remain unclear. Therefore, the behavior of NCPs precipitation induced by HCPEB surface modification and its influence on the tribological properties of WC-Co hard alloy will be investigated systematically in the present project, including the precipitation threshold and microstructure evolution of NCPs, the structure transformation and movement of NCPs during friction and wear processes, etc. Basing on the experimental and theoretical analyses, the precipitaion mechanism and self-lubrication function of NCPs in the surface layer of WC-Co hard alloy induced by HCPEB surface modification will be revealed together with the optimum treating parameters accordingly. This work will be of important significance to the theories of high-temperature, non-equilibrium phase transformation of WC intermetallic compound and also the exploration of effective methods for WC-Co hard alloy surface modification.

现代加工制造业对WC-Co硬质合金的材料性能提出更高要求，表面硬度低、耐磨性差是现有硬质合金工具(WC晶粒>0.3μm)的主要问题。高能束表面改性能有效提高WC-Co硬质合金的使役性能，且无膜基结合强度问题，但由于设备因素使应用研究受到限制。申请人利用自行研制的强流脉冲电子束装置进行WC-Co硬质合金表面改性，发现WC晶粒原位弥散析出纳米碳粒子，将耐磨性提高十几倍，但对于碳粒子的析出及减磨机制尚不清楚。基于此，本项目拟针对强流脉冲电子束辐照WC-Co硬质合金表面纳米碳粒子的析出行为及其对摩擦学性能的影响进行深入研究，包括WC晶粒的纳米碳粒子析出条件和组织演化规律；纳米碳粒子在摩擦磨损过程中的结构变化和迁移行为等，揭示WC-Co硬质合金表面纳米碳粒子的析出和自润滑机制，实现减磨功能优化。本研究对发展WC金属化合物高温非平衡相变理论，开发新型WC-Co硬质合金表面改性技术具有重要的指导意义。

现代加工制造业的迅猛发展对硬质合金工具的使役性能提出更高要求，有效提高WC-Co硬质合金工具的表面硬度和耐磨性是满足高速、高效和高精度加工需求的关键。本项目利用自行研制的HOPE-I型强流脉冲电子束装置进行WC-Co硬质合金表面改性实验，分析发现改性表面分布有微米量级熔坑和细密集烧蚀斑点，表层WC晶粒逐步熔化，形成纳米级精细快凝组织， 6次脉冲处理样品层纳米碳粒子数量最多且分布均匀，20次脉冲处理熔深达到1.5 µm，碳粒子集中于重熔层底部。原始WC相发生分解，新相WC1-x成为表面改性层的主要成分，并呈WC1-x（111）择优取向。随着脉冲次数增加，WC相持续减少，WC1-x（111）择优取向逐渐减弱，石墨相在6次脉冲辐照条件下达到最强，并观察到少量三元相Co3W3C和Co6W6C。利用TEM分析证实改性表层的颗粒状析出物为纳米石墨球。对强流脉冲电子束辐照WC-Co硬质合金表层温度场进行模拟计算，表层最高温度~5700 K，表层升温速率108-1010 K/s，冷却速度107-108 K/s。结合WC晶粒非均匀熔化特征和W-C-Co成分相图，分析认为未均质熔化WC晶粒通过逆包晶（WC→gra + liq）形成尺寸~50 nm的纳米碳粒子，WC晶粒外层熔化与Co粘结相发生互扩散，并在富Co元素区域形成~100 nm纳米碳粒子。测试结果表明，表面显微硬度随脉冲次数呈线性增加，35次脉冲处理样品表面显微硬度达到3128.5Hv，较原始样品提高80%，10次脉冲处理表面摩擦系数减小到0.29，约为原始试样的三分之一，6次脉冲处理样品磨损率最低1.62×10-4 mm3/min，降为原始样品的40%，测试表明摩擦过程使部分碳粒子碾压延展形成具有润滑作用的碳膜，过多脉冲处理会出现改性层大片磨损剥离现象。本项目研究工作揭示了强流脉冲电子束辐照WC-Co硬质合金表层非平衡相变规律及其摩擦学性能指标，对于探索WC-Co硬质合金表面改性有效途径具有重要的理论和实践指导意义，所获得的强流脉冲电子束WC-Co硬质合金表面改性工艺装备可以用于机加工刀模具、矿山采掘部件，以及有耐磨防腐特殊要求部件的表面处理，为加快我国工业发展和科技进步做出贡献。