Preparation and properties research of Cu-based electrical contact composites containing Cr and NbSe2
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摘要: 采用复压复烧工艺制备了含不同质量分数Cr和NbSe2的铜基电接触复合材料，利用光学电子显微镜、X射线衍射仪、硬度计、扫描电子显微镜等设备研究了铜基复合材料力学、电学和电摩擦学性能。结果表明，铜基复合材料的密度随着NbSe2含量的增多而增高，硬度和断裂强度随着Cr含量的增多而提高；Cr含量高的铜基复合材料磨痕表面极易生成CuO纳米片，改善了材料的摩擦性能，但降低了电学性能；含适当比例Cr和NbSe2的铜基复合材料有着较好力学、电学性能，且因NbSe2润滑膜和CuO纳米球的协同作用，改善了材料的摩擦性能。Abstract: The Cu-based electrical contact composite materials containing Cr and NbSe2 in the different mass fractions were prepared by repressing and reburning method in this study, and the mechanical, electrical, and tribological properties of copper-based electrical contact composites were investigated by optical electron microscope, X-ray diffractometer, hardness tester, and scanning electron microscope. The results show that, the density of the copper-based composites increases with the increase of NbSe2 content, while the hardness and fracture strength increase with the increase of Cr content. The copper-based composites with the higher Cr content can easily generate the CuO nanoplates on the wear scar surface, which improves the tribological properties but reduces the electrical properties of the copper-based composite materials. Consequently, the copper-based composites containing Cr and NbSe2 in the appropriate mass fraction show the better mechanical and electrical properties. Moreover, the tribological properties of the copper-based composites are also improved due to the synergistic effect of the NbSe2 lubricating film and CuO nanospheres.
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锆酸钙材料（CaZrO3）具有优秀的抗水化性能、高熔点及良好的抗热震性能[1-5]，拥有广阔的应用前景，由于自然界中不存在天然的CaZrO3，研究锆酸钙材料的合成就显得非常必要。制备CaZrO3的方法主要包括高温固相反应法、共沉淀法、溶胶-凝胶法、燃烧法和水热法等[6-8]，高温固相法由于工艺简单、生产成本较低和生产量大等优点被人们广泛使用，但这种方法存在烧结温度高、制备锆酸钙致密性差等缺点。为了解决这些问题，研究者们在制备锆酸钙材料过程中向物系添加少量稀土氧化物、Al2O3、SiO2、CuO等添加剂，用于促进锆酸钙在低温下的烧结致密化；这些添加剂虽然可以起到促进锆酸钙材料烧结致密性的作用[9-11]，但也会带来外来物质，降低CaZrO3高温使用性能。
CaCO3作为制备CaZrO3的添加剂在高温下分解生成CaO，不会对CaZrO3产生污染；同时，由于CaCO3和制备原料Ca(OH)2分解温度不同，产生CaO晶体顺序不同，可以对CaO晶体质点的扩散产生影响。故本文考虑向锆酸钙材料中添加少量CaCO3微粉，利用分解温度不同，生成CaO晶体顺序不同，促进CaZrO3烧结致密性，降低锆酸钙烧结温度。
1. 实验材料及方法
1.1 实验材料
以天津市科密欧化学试剂有限公司生产的分析纯Ca(OH)2和天津市光复精细化工研究生产的m-ZrO2为主要原料（平均粒度为7.4 μm和4.5 μm，纯度大于99%），实验中添加的CaCO3微粉为高纯微粉，纯度大于99%，其粒度分布如图 1示。可以看出，CaCO3微粉粒度较小，主要粒度分布在10 μm左右，D50为6 μm，D90为24 μm。
1.2 实验过程及方法
将Ca(OH)2和m-ZrO2按摩尔比1:1称量，等量分成五组，每组混合粉末中依次加入质量分数为0%、2%、4%、6%、8%和10%CaCO3微粉，再用卧式球磨机混合12 h，经过FLS手动四柱油压机在200 MPa压力下将混合粉末压制成ϕ20 mm圆柱试样，再用硅钼棒高温烧结炉在1600 ℃加热并保温3 h后随炉冷却到常温以备性能检测。
烧结前将压好的试样放置在烘箱内110 ℃下保温24 h，取出冷却至常温，测量其高度（L0）；试样经高温煅烧，冷却到常温后测量其烧后高度（L1），根据式（1）计算试样烧结前后线变化率（ΔLd）。
$$ \Delta {L_{\rm{d}}} = \left[ {\left( {{L_1} - {L_0}} \right)/{L_0}} \right] \times 100\% $$ (1) 利用阿基米德排水法检测试样煅烧后的体积密度和显气孔率[12]。煅烧后试样经切割、抛光及热处理后，采用扫描电子显微镜（scanning electron microscope，SEM）观察其组织形貌，使用X射线衍射仪（X-ray diffractometer，XRD）对其进行物相分析。
2. 结果与讨论
2.1 烧结性能
图 2为烧结前后试样线变化率，从图 2可以看到，CaCO3微粉加入会改变试样线变化率。没有添加CaCO3微粉时，试样烧结前后线变化率为8.23%；当添加CaCO3微粉质量分数小于8%时，随CaCO3微粉添加量增大，试样烧结前后线变化率逐渐增大；当加入CaCO3微粉质量分数为8%时，试样收缩率达到最大值，为14.89%；继续增大CaCO3微粉添加量，试样烧结前后线变化率呈降低趋势。
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图 2 线变化率与添加CaCO3微粉质量分数的关系Figure 2. Relationship between shrinkage and the CaCO3 addition content by mass图 3为高温煅烧后制备的锆酸钙体积密度和显气孔率，由图 3可以看到，CaCO3微粉的引入对制备的锆酸钙烧结性能产生影响。当没有添加CaCO3微粉时，制备的锆酸钙体积密度为3.4 g·cm-3，显气孔率为14.5%；随CaCO3质量分数增加，制备锆酸钙体积密度逐渐增加，显气孔率逐渐减小；当CaCO3微粉添加量为8%时，制备锆酸钙的体积密度最大，为4.02 g·cm-3，显气孔率最小，为8.6%；当CaCO3质量分数继续增大时，锆酸钙的体积密度开始降低，显气孔率反增大。
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图 3 烧结试样体积密度、显气孔率与添加CaCO3质量分数的关系Figure 3. Relationship of bulk density, apparent porosity, and CaCO3 addition content by mass of sintering samples图 4为添加质量分数10%CaCO3制备样品的X射线衍射图谱，从图中可以看出，样品经1600 ℃保温3 h后主要物相为CaZrO3以及少量CaZr4O18。
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图 4 添加质量分数10%CaCO3微粉制备样品的X射线衍射图谱Figure 4. XRD patterns of samples add by CaCO3 powders in the mass faction of 10%2.2 材料微观结构
图 5为添加不同质量分数CaCO3微粉的样品在1600 ℃烧后放大10000倍的扫描电子显微组织结构图。从图 5可以看出，CaCO3微粉质量分数小于8%时，随CaCO3微粉添加量的增大，试样致密性逐渐增加，锆酸钙晶粒尺寸逐渐变大，且晶体发育越来越均匀；当CaCO3微粉质量分数为8%时，锆酸钙晶粒尺寸最大，试样中基本无封闭气孔；当CaCO3微粉质量分数继续增大时，样品中出现封闭气孔，致密性变差，锆酸钙晶粒尺寸有变小趋势。
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图 5 添加不同质量分数CaCO3微粉的锆酸钙试样在1600 ℃烧结后扫描电子显微组织形貌：（a）0%；（b）2%；（c）4%；（d）6%；（e）8%；（f）10%Figure 5. SEM micrographs of sintered CaZrO3 samples at 1600 ℃ added by CaCO3 powders in different mass fractions: (a) 0%; (b) 2%; (c) 4%; (d) 6%; (e) 8%; (f) 10%利用图象处理软件对图 5进行定量晶体大小测定，获得锆酸钙的平均晶粒尺寸，见表 1。可以发现，没有引入CaCO3微粉时，样品中锆酸钙晶粒尺寸最小为4.08 μm；随CaCO3微粉质量分数增大，锆酸钙晶粒尺寸逐渐增大；当CaCO3微粉质量分数为8%时，锆酸钙晶粒尺寸达到最大，为5.45 μm；当CaCO3微粉质量分数量继续增大时，锆酸钙晶粒尺寸反而变小。
表 1 样品中CaCO3质量分数与锆酸钙晶粒直径的关系Table 1. Relationship between CaZrO3 particle diameter and CaCO3 addition content by massCaCO3质量分数/% 0 2 4 6 8 10 CaZrO3晶粒直径/μm 4.08 4.43 4.88 5.08 5.45 5.21 2.3 促烧机理
为了分析CaCO3微粉对锆酸钙烧结性能的影响，选取添加质量分数8%CaCO3微粉的试样，分别在500、600、700、800、900、1000及1100 ℃下保温3 h，分析在各个温度下烧后试样物相组成。图 6为试样在不同温度烧结后X射线衍射图谱。可以看出，试样经过500 ℃保温3 h后，物相组成没有太大变化；经过600 ℃保温3 h后，物相中开始有少量CaO出现，这是因为Ca(OH)2分解为CaO温度为580 ℃左右[13]；当试样在700、800 ℃保温3 h后，Ca(OH)2质量分数逐渐减少，衍射峰逐渐减弱，CaO质量分数逐渐增大，衍射峰峰强逐渐增强，CaCO3衍射峰强在700 ℃之前逐渐增强，这是因为随烧结温度的升高，CaCO3晶粒发育越来越充分，烧成温度达到800 ℃时，CaCO3衍射峰强开始减弱，说明CaCO3开始分解为CaO；烧结温度为900 ℃时，CaCO3衍射峰逐渐减弱，CaO峰强增加迅速，这是因为CaCO3理论分解温度为850 ℃左右[14]，分解生成高活性的CaO微晶均匀附着在Ca(OH)2分解形成CaO晶体表面，从而有利于CaO晶体扩散，可以促进CaO晶体长大，提高了CaO晶体的均匀性和生长致密性；继续升高烧结温度，CaCO3衍射峰强逐渐减弱乃至消失。
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图 6 添加质量分数8%CaCO3试样在不同温度烧结后X射线衍射图谱Figure 6. XRD patterns of samples sintered at different temperatures add by CaCO3 powders in the mass faction of 8%当烧结温度达到900 ℃时，物相中开始出现CaZrO3衍射峰，说明开始生成CaZrO3。随烧结温度的提高，CaZrO3衍射峰强增加迅速，一部分原因是因为温度升高，CaZrO3迅速长大，另一部分原因是因为CaCO3分解CaO微晶附着在Ca(OH)2分解形成的CaO晶体表面，促进CaO晶体长大，为高温下CaO和ZrO2反应生成CaZrO3奠定基础。但添加过多的CaCO3微粉时，由于CaCO3在分解过程中产生过量CO2气体逸出形成大量的气体孔洞，不利于质点的迁移，导致烧结性能变差。
3. 结论
（1）添加少量CaCO3微粉有利于锆酸钙烧结致密性。没有添加CaCO3微粉时，烧结温度为1600 ℃，锆酸钙体积密度为3.40 g·cm-3，显气孔率为14.5%；添加质量分数8%CaCO3微粉时，锆酸钙体积密度为4.02 g·cm-3，显气孔率为8.6%。
（2）添加少量CaCO3微粉有利于锆酸钙晶粒长大。烧结温度为1600 ℃，无添加CaCO3微粉时，锆酸钙晶粒尺寸为4.08 μm；添加质量分数8%CaCO3微粉时，锆酸钙晶粒尺寸为5.45 μm。
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图 2 铜基试样的力学性能：（a）显微硬度；（b）断裂强度
Figure 2. Mechanical properties of Cu-based samples: (a) microhardness; (b) fracture strength
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图 3 铜基试样光学显微形貌：（a）Se1；（b）Se3；（c）Se5
Figure 3. Optical micrographs of Cu-based samples: (a) Se1;(c) Se3;(c) Se5
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图 4 Se1试样的电摩擦学性能：（a）摩擦系数和电流变化；（b）显微形貌；（c）显微形貌框型区域能谱分析
Figure 4. Electrical tribological properties of sample Se1:(a) COF and I; (b) SEM images; (c) EDSanalysis at the box type area in SEM
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图 5 Se1试样磨痕的光电子能谱分析
Figure 5. X-ray photoelectron spectroscopy (XPS) analysis of sample Se1
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图 6 Se2试样的电摩擦学性能：（a）摩擦系数和电流变化；（b）显微形貌；（c）显微形貌框型区域能谱分析
Figure 6. Electrical tribological properties of sample Se2:(a) COF and I; (b) SEM images; (c) EDSanalysis at the box type area in SEM
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图 7 Se3试样的电摩擦学性能：（a）摩擦系数和电流变化；（b）显微形貌；（c）显微形貌框型区域能谱分析
Figure 7. Electrical tribological properties of sample Se3:(a) COF and I; (b) SEM images; (c) EDSanalysis at the box type area in SEM
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图 8 Se4试样的电摩擦学性能：（a）摩擦系数和电流变化；（b）显微形貌；（c）显微形貌框型区域能谱分析
Figure 8. Electrical tribological properties of sample Se4:(a) COF and I; (b) SEM images; (c) EDS analysis at the box type area in SEM
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图 9 Se5试样的电摩擦学性能：（a）摩擦系数和电流变化；（b）显微形貌；（c）显微形貌框型区域能谱分析
Figure 9. Electrical tribological properties of sample Se5:(a) COF and I; (b) SEM images; (c) EDSanalysis at the box type area in SEM
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图 10 Se6试样的电摩擦学性能：（a）摩擦系数和电流变化；（b）显微形貌；（c）显微形貌框型区域能谱分析
Figure 10. Electrical tribological properties of sample Se6:(a) COF and I; (b) SEM images; (c) EDS analysis at the box type area in SEM
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图 11 电摩擦测试前后试样电阻率的变化
Figure 11. Resistivity of the Cu-based composite samples before and after the electric friction test
表 1 铜基复合材料试样组成成分及物理性能
Table 1 Composition and physical properties of the copper-based composite samples
试样编号 成分质量分数/% 密度, ρ/(kg·m-3) 相对密度，ρr/% 表面粗糙度，Ra/μm Cu NbSe2 Cr Sel 65 0 35 6.96×103 84.9 0.217 Se2 65 10 25 7.13×103 88.1 0.202 Se3 65 15 20 7.24×103 90.2 0.214 Se4 65 20 15 7.39×103 92.7 0.186 Se5 65 25 10 7.42×103 94.0 0.175 Se6 65 35 0 7.26×103 93.3 0.193 
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表 2 金属及其金属氧化物的电阻率
Table 2 Electrical resistivity of the metals and metal oxides
材料 Nb NbO Cr Cr2O3 Cu CuO Cu20 NbSe2 电阻率/(Ω·m) 12.5 × 10-8 10-7 12.9 × 10-8 — 1.534 × 10-8 10~50 10~50 5.35 × 10-6
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