B4C陶瓷的硬度极高但是断裂韧性较低，影响其应用。Hwang等[6]采用放电等离子烧结技术制备的单相B4C陶瓷，其断裂韧性仅为2.0 MPa·m1/2。陈威等[7]热压烧结制备的单相B4C陶瓷，其断裂韧性为2.58 MPa·m1/2。Xu等[8]发现，添加5% (质量分数)Al-Si二元烧结助剂的无压烧结B4C陶瓷，其断裂韧性为2.9 MPa·m1/2。Zhang等[9]使用3% (质量分数)纳米炭黑作为烧结助剂无压烧结制备的B4C陶瓷，其断裂韧性为3.2 MPa·m1/2。较低的断裂韧性限制了B4C陶瓷的应用。在B4C陶瓷基体中添加第二相，可使B4C陶瓷增韧。Baharvandi和Hadian[10]引入30% (质量分数)TiB2无压烧结制备的B4C-TiB2复合陶瓷，其断裂韧性为3.4 MPa·m1/2。Zhang等[11]热压烧结制备的B4C-50% (质量分数)SiC复合陶瓷，其断裂韧性为4.6 MPa·m1/2。
氧化铝(Al2O3)的熔点、硬度和强度较高[12,13]，常作为增韧相添加到陶瓷基体中。She等[14]发现，在单相SiC陶瓷添加5% (质量分数)的Al2O3可使其断裂韧性提高到5.41 MPa·m1/2。Tekeli[15]指出，在ZrO2陶瓷中引入10% (质量分数)的Al2O3可使其断裂韧性由1.5 MPa·m1/2提高到2.4 MPa·m1/2。Al2O3作为烧结助剂能促进B4C陶瓷的烧结。Lee和Kim[16]指出，添加3% (质量分数)的Al2O3能促进无压条件下B4C陶瓷的烧结。Al2O3烧结助剂不仅能促进B4C陶瓷的烧结，还能改善B4C陶瓷的微观结构和提高其力学性能。其原因是，Al2O3能为B4C烧结提供液相扩散和钉扎效应，因而抑制了B4C晶粒的异常长大[17]；在B4C中添加Al2O3虽然因其热膨胀失配产生残余应力不利于其他力学性能(如弯曲强度)，但是却有利于提高其断裂韧性[18]。鉴于此，本文用热压烧结工艺制备不同Al2O3含量的B4C-Al2O3复合陶瓷，研究Al2O3对其断裂韧性的影响并揭示其增韧机理。
1 实验方法
1.1 B4C-Al2O3 复合陶瓷的制备
制备B4C-Al2O3复合陶瓷的原料有：B4C粉末(D50 = 0.21 μm，纯度94.4%)和Al2O3粉末(D50 = 2.36 μm，纯度99.99%)。为了与在相同温度下烧结的单相B4C陶瓷对比，还以B4C粉末为原料，以Al2O3粉末和Y2O3粉末(D50 = 1.32 μm，纯度99.999%)为烧结助剂制备了B4C陶瓷。
根据表1中的成分，将B4C和Al2O3或B4C和烧结助剂Al2O3、Y2O3粉末混合后进行高能球磨、干燥、过筛，得到混合粉体。将混合粉体装入直径为50 mm的石墨模具并置于ZT-63-20YB型高温热压烧结炉在氩气气氛中烧结。烧结工艺为：温度为1950℃，压力为30 MPa，保温保压时间为1 h。将烧结好的圆盘试样打磨、切割和抛光，制备测试断裂韧性的试样，其尺寸分别为22 mm × 22 mm × 3 mm (压痕法)和4 mm × 3 mm × 36 mm (单边切口梁法)。
表1 B4C-Al2O3复合陶瓷的成分设计(质量分数，%)
Table 1
| B4C | Al2O3 | Al2O3 + Y2O3 | |
|---|---|---|---|
| A0 | 91 | 0 | 9 |
| A1 | 90 | 10 | 0 |
| A2 | 80 | 20 | 0 |
| A3 | 70 | 30 | 0 |
| A4 | 60 | 40 | 0 |
1.2 性能表征
用D-8 Advanced型X射线衍射分析仪(XRD)分析试样的物相组成；用Supra35型扫描电子显微镜(SEM)和Talos F200X G2型透射电子显微镜(TEM)观察试样的微观组织结构。分别根据TEM图片和SEM图片统计B4C晶粒和Al2O3晶粒的平均晶粒尺寸。用Archimedes排水法测定试样的密度并计算相对密度；用压痕法测试试样的断裂韧性；用Q10A型维氏硬度计和压痕法测试试样的断裂韧性，载荷5 kg，保压时间15 s，并记录维氏硬度。用CMT4204型万能试验机和单边切口梁法测试试样的断裂韧性，加载速度0.5 mm/min，支点跨距30 mm，切口深度2 mm。用压痕法和单边切口梁法测试试样断裂韧性，计算公式分别为[19,20]
式中KIC为断裂韧性(MPa·m1/2)，E为弹性模量(GPa)，H为维氏硬度(MPa)，P为载荷(N)，c为裂纹半长(mm)，P为断裂载荷(N)，S为支点跨距(mm)，a为切口深度(mm)，B为试样宽度(mm)，W为试样高度(mm)，
为试样的形状因子。
2 结果和讨论
2.1 B4C-Al2O3 复合陶瓷的物相组成
图1
2.2 B4C-Al2O3 复合陶瓷的相对密度和晶粒尺寸
表2列出了B4C-Al2O3复合陶瓷的相对密度和晶粒尺寸。在1950℃烧结原位生成的烧结助剂Y3Al5O12促进了单相B4C陶瓷的烧结，使其相对密度提高。B4C-Al2O3复合陶瓷的相对密度随着Al2O3加入量由10%提高到30%而增加，Al2O3的加入量再增加基本上保持不变。Al2O3的熔点(2054℃)低于B4C的熔点(2450℃)。根据B4C-Al2O3的二元相图，引入Al2O3能降低B4C陶瓷的烧结温度。因此，Al2O3作为第二相可促进B4C陶瓷的烧结，加入量Al2O3 ≥ 20%的B4C-Al2O3复合陶瓷其致密度较高。
表2 B4C-Al2O3复合陶瓷的相对密度和晶粒尺寸
Table 2
| Relative density / % | Grain size / μm | |||
|---|---|---|---|---|
| B4C | Al2O3 | Avg. | ||
| A0 | 98.4 | 1.07 | - | - |
| A1 | 88.9 | 1.05 | 1.56 | 1.08 |
| A2 | 96.1 | 1.01 | 1.22 | 1.04 |
| A3 | 98.8 | 0.75 | 1.47 | 0.90 |
| A4 | 98.5 | 0.81 | 1.63 | 1.05 |
从表2还可以看出，B4C-Al2O3复合陶瓷中Al2O3的平均晶粒尺寸为1.22~1.63 μm，B4C的平均晶粒尺寸为0.75~1.05 μm。Al2O3的平均晶粒尺寸小于初始Al2O3粉末粒径的原因是，高能球磨减小了Al2O3粉末的晶粒。B4C晶粒和Al2O3晶粒的尺寸均先减小后增大，且Al2O3的晶粒尺寸大于B4C的晶粒尺寸。计算结果表明，随着Al2O3含量的提高B4C-Al2O3复合陶瓷的平均晶粒尺寸先减小后增大。但是在总体上，B4C-Al2O3复合陶瓷的平均晶粒尺寸相差不明显，其范围为0.90~1.08 μm。Al2O3作为第二相引入B4C陶瓷抑制了B4C晶粒的长大，因此B4C的晶粒尺寸随着Al2O3含量的提高而减小。但是，Al2O3含量为40%的A4试样其表面局部出现呈树枝晶的Al2O3晶粒(图2)。这是液相再结晶的一种凝固形式[21]，表明A4试样中局部出现了液相。局部的液相促进了B4C晶粒和Al2O3晶粒的长大，使A4试样的平均晶粒尺寸略有增大。但是只在局部有液相，A4试样的平均晶粒尺寸并没有明显的增大。图3给出了A3试样典型的TEM照片，可见B4C晶粒大部分呈矩形或椭圆形，Al2O3晶粒的形状不规则。
图2
图3
图3
A3试样中晶粒的形貌
Fig.3
Grain morphology of A3 specimen (a) typical bright-field TEM micrograph and (b) EDS analysis of O element in Fig.3a
2.3 B4C-Al2O3 复合陶瓷的断裂韧性
图4给出了用压痕法和单边切口梁法测出的B4C-Al2O3复合陶瓷的断裂韧性。可以看出，用压痕法测试时，A1试样的断裂韧性明显增大。在本文的实验条件下，A1试样的相对密度较低，材料中有较多的气孔。压头压入较多气孔的陶瓷表面时，一部分能量用于陶瓷致密化，其余的能量驱使裂纹扩展[22]，因此裂纹的扩展路径较短。由
图4
图4
B4C-Al2O3复合陶瓷的断裂韧性
Fig.4
Fracture toughness of B4C-Al2O3 composite ceramics
图5
图5
A1和A3试样断裂韧性的表面形貌
Fig.5
Surface morphologies of A1 and A3 specimens tested for fracture toughness via IF (a) A1 and (b) A3
2.4 B4C-Al2O3 复合陶瓷的增韧机理
陶瓷的晶粒尺寸也是影响其断裂韧性的重要因素[25]。晶粒较细的陶瓷中晶界较多，晶界吸收裂纹扩展能使裂纹偏转，不易出现穿晶断裂，从而抑制了裂纹的扩展，有利于提高陶瓷的断裂韧性[23,26]。Moshtaghioun等[22]发现，随着用放电等离子烧结的B4C-SiC陶瓷中晶粒尺寸的减小，其断裂韧性增大。本文制备的B4C-Al2O3复合陶瓷(A1、A2、A3、A4)的平均晶粒尺寸先减小后增大。根据传统理论，B4C-Al2O3复合陶瓷的断裂韧性应该先增大后减小，但是实际的断裂韧性的变化规律并不与之相符。同时，A1、A2、A4试样的平均晶粒尺寸差别不明显，但是其断裂韧性值的差异较大。这表明，本文制备的B4C-Al2O3复合陶瓷，其晶粒尺寸并不是B4C-Al2O3陶瓷增韧的主要因素。
图6给出了用单边切口梁法测试的B4C-Al2O3复合陶瓷断裂韧性试样的断口形貌。可以看出，A0试样以穿晶断裂为主(图6a)。A1试样内的气孔较多(图6b)，与表2中的数据相符。根据图6b~e，在B4C-Al2O3复合陶瓷内形成了解理结构(图中的圆圈)。对图6f中B点的EDS分析结果表明，出现解理结构的物相为Al2O3。在晶体结构为三方晶系的Al2O3晶体中，(10
图6
图6
B4C-Al2O3复合陶瓷用单边切口梁法测试断裂韧性试样的断口形貌图
Fig.6
Fracture surfaces of B4C-Al2O3 composite ceramics tested for fracture toughness by SENB (a) A0, (b) A1, (c) A2, (d) A3, (e) A4, (f) EDS analysis of point B in Fig.6 (e)
同时，B4C的热膨胀系数为6.5 × 10-6℃-1，Al2O3的热膨胀系数为8.8 × 10-6℃-1，两者的差别较大。从图7a给出的A4试样的高分辨透射电镜照片可见，B4C晶粒与Al2O3晶粒直接连接，相界没有非晶相，进一步证明图2中的液相只出现在局部。在试样烧结后的冷却过程中，基体相B4C与第二相Al2O3热膨胀失配产生了残余应力，B4C晶粒内部为压应力，Al2O3晶粒内部为张应力(图7b)。因此，当裂纹延伸至B4C晶粒内时裂纹的扩展受到了抑制。同时，在B4C晶粒与Al2O3晶粒的相界产生了张应力。虽然干净的相界使两相晶粒牢固连接而不易使裂纹沿晶扩展[28,29]，但是B4C-Al2O3复合陶瓷中B4C晶粒与Al2O3晶粒相界的张应力在一定程度上弱化了相界的结合，使部分裂纹沿晶扩展导致裂纹偏转，从而提高了B4C-Al2O3复合陶瓷的断裂韧性。从图6中试样的断口形貌可见，B4C-Al2O3复合陶瓷的断裂形式从A0试样的穿晶断裂为主逐渐过渡到A2、A3、A4试样的穿晶和沿晶混合断裂模式。这个结果表明，B4C晶粒与Al2O3晶粒相界的张应力使部分裂纹沿晶扩展。
图7
图7
A4试样的高分辨透射电镜照片和B4C-Al2O3复合陶瓷中应力分布示意图
Fig.7
HRTEM image of A4 specimen (a) and schematic diagram of stress distribution in B4C-Al2O3 composite ceramics (b)
3 结论
Al2O3作为第二相引入B4C陶瓷能提高复合陶瓷的断裂韧性，其增韧机理是Al2O3晶粒在裂纹扩展过程中形成的解理结构延长了裂纹扩展的路径。虽然B4C晶粒内部的压应力有利于抑制裂纹的扩展，但是Al2O3晶粒与B4C晶粒间因热膨胀失配产生的张应力能在一定程度上弱化相界的结合，使部分裂纹在扩展过程中沿晶扩展而偏转，从而提高了B4C-Al2O3复合陶瓷的断裂韧性。Al2O3加入量为40%的B4C-Al2O3复合陶瓷，其断裂韧性最大(4.96 MPa·m1/2)。
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