基于不含氮原料制备的脱β层梯度硬质合金的形成机理与生长动力学研究

The graded cemented carbide with cubic carbide free layer(CCFL) was prepared usually by employing raw material powders containing hard and brittle cubic nitrides or carbonitrides, which may weaken the compactness and mechanical property of the graded alloy. In this project proposed, by controlling change of sintering atmosphere during sintering process, appropriate degree nitridation and denitrification reaction process happen successively in surface zone of green compact, as a result, the graded cemented carbide with CCFL is prepared successfully with raw material powders free nitrogen. Moreover, for revealing formation mechanism of the graded structure and controlling the growth rate of CCFL, the effect of preparation process parameters on gradient structure and mechanical properties are studied. Through preparation of the graded cemented carbides in different sintering stages by means of interrupt sintering and rapid cooling method, the dynamic variation rule of microstructure, phase composition and element distribution during throughout sintering process is concluded, and the formation mechanism of the graded layer is revealed. The physical models of CCFL is founded and its growth kinetics equations is derived based upon the outward diffusion of nitrogen, the inward diffusion of titanium, the coupled diffusion of nitrogen and titanium respectively. Finally, the nitriding reaction model is founded, the equilibrium condition and reaction kinetics equations are derived from investigation into the nitriding reaction process of cubic carbide in surface zone. The successful implementation of this project can provide theoretical guidance for preparation process of the graded cemented carbide with CCFL by employing raw material powders free nitrogen, and has great significance to further improve the performances of the graded cemented carbide.

通常脱β层梯度硬质合金的制备需在原料中预先添加硬、脆的含氮相，容易降低合金的致密度及力学性能。本项目基于不含氮的硬质合金原料，在烧结过程中通过控制烧结气氛的变化，使压坯表层先后发生适当程度地氮化、脱氮反应，成功制备脱β层梯度硬质合金。为揭示梯度结构的形成机制，控制梯度层的生长速率，研究制备工艺参数对梯度结构及合金性能的影响规律；通过打断烧结急速冷却的方法，制备不同烧结阶段下的梯度硬质合金，分析烧结过程中合金微观组织、相组成的动态变化及各元素的动态迁移规律，揭示梯度层的形成机理；分别基于氮向外扩散、钛向内扩散及氮钛耦合扩散机制建立脱β层形成的物理模型及生长动力学方程；分析合金表层立方碳化物相的氮化反应过程，建立氮化反应模型，推导氮化反应的平衡条件与反应动力学计算方程。本项目的成功实施为基于不含氮原料制备脱β层梯度硬质合金提供理论指导，对于进一步提高脱β层梯度硬质合金的性能具有重要意义。

本项目基于不含TiN、Ti(C,N) 等含氮立方相的初始混合料，开发了一种微压氮化+真空脱氮烧结工艺制备脱β层梯度硬质合金的新技术。该技术在极大程度上规避了传统脱β层梯度硬质合金的制备中预先添加的含氮相提前分解导致合金致密度及力学性能降低等问题，有望明显提高脱β层梯度硬质合金的性能。.首先分析了制备工艺参数对合金性能及梯度结构的影响规律，结果表明：合金的力学性能除了受初始成分影响外，还取决于合金内部的孔隙度及游离碳的分布，在合金内部存在较大孔隙及游离碳时，合金抗弯强度明显降低。脱β层的厚度随着钴含量的增加及烧结温度的提高而增大；但与直接添加Ti(C,N)的原料相比，在同样的烧结工艺下，采用不含氮的原料制备的梯度硬质合金的脱β层厚度相对较小。试验结果证实：所采用的氮气分压(0.5 kPa)足以保证氮化反应的持续进行，脱氮烧结工艺1450 ℃、2 h 是较为合适的烧结参数，它既能保证形成一定厚度的脱β层，又能避免WC相的长大，有利于保证合金的力学性能。.另外，还深入分析了梯度层的形成机理，认为合金表层的梯度层的形成是由缺立方相层的形成及富钴层的形成两个同时进行的过程共同决定的。采用传统的氮钛耦合扩散机制解释缺立方相层的形成过程；首次提出了一种新的富钴层的形成机制，认为富钴层的形成是由钴元素的空位扩散和液相迁移的联合作用机制导致的，并且冷却过程中发生的液相迁移过程进一步导致了富钴层内部不同区域之间钴含量的差异。.最后，在上述形成机理研究的基础上，分别建立了氮的向外扩散、钛向内扩散及氮钛耦合扩散等环节控制的脱β层的生长动力学方程及氮化过程的反应动力学方程。本项目的成功实施为脱β层梯度硬质合金的制备工艺提供理论指导，对于制备高性能脱β层梯度硬质合金、进而延长数控涂层刀片的服役寿命具有重要意义。