热处理对硬质合金表面Co转变及生长金刚石涂层的影响
Effect of Heat Treatment on Co Conversion and Growth of Diamond Coating on Cemented Carbide Surface

作者: 曾己伟 :华南理工大学材料科学与工程学院,广东 广州;广州汇专绿色工具有限公司,广东 广州; 李伟秋 :广州汇专绿色工具有限公司,广东 广州; 彭继华 :华南理工大学材料科学与工程学院,广东 广州;

关键词: 热处理硬质合金金刚石涂层THot Treatment Cemented Carbide Diamond Coating

摘要: 利用热处理将YG6硬质合金表层的Co转变为Co化合物,然后采用自行研发的热丝CVD设备在热处理过后的YG6硬质合金表面沉积约2.2 μm厚度的金刚石涂层。采用X射线衍射仪、扫描电子显微镜、拉曼光谱仪、X射线光电子能谱仪、洛氏硬度计对热处理过后的YG6硬质合金表层物相与组织、金刚石涂层质量进行表征。分析结果表明,采用热处理可将硬质合金表层一定深度内的Co完全转化为Co3W。该转化层可阻挡Co扩散至金刚石涂层,有效促进金刚石涂层的形核和生长。热处理气氛中的H2含量过低使部分Co被氧化,将降低金刚石涂层质量,导致菜花状纳米金刚石涂层形成,氧化严重时甚至会使金刚石涂层无法形成。

Abstract: The Co on the surface of YG6 cemented carbide was transformed into Co compound by heat treat-ment. Then the diamond coating of about 2.2 μm thickness was deposited on the surface of YG6 carbide after heat treatment by using the self-developed hot filament CVD equipment. X-ray dif-fractometer, scanning electron microscope, Raman spectrometer, X-ray photoelectron spectrometer and Rockwell hardness tester were used to characterize the surface phase and organization of YG6 carbide and the quality of diamond coating after heat treatment. The analysis results show that the Co in certain depth of surface layer of cemented carbide can be completely transformed into Co3W by heat treatment. The conversion layer can prevent Co diffusion to diamond coating and effectively promote the nucleation and growth of diamond coating. The low H2 content in the heat treatment atmosphere leads to the oxidation of part of Co, which will reduce the quality of diamond coating and lead to the formation of cauliflower-like nano-diamond coating. Severe oxidation will even make diamond coating unable to form.

1. 介绍

当前金刚石涂层硬质合金刀具在先进制造业极具前景。金刚石涂层刀具失效多来源于涂层剥落 [1] [2] [3]。剥落主要原因有两个:1) Co石墨催化效应;2) 热膨胀系数不匹配 [4] [5] [6] [7]。目前已采用不少预处理方法,诸如酸蚀 [8]、过渡层等等 [9] - [14],以及采用渗硼将硬质合金表面Co转变为Co-W-B或Co-B化合物,可改善涂层结合力 [15] [16]。虽然采用脱碳热处理可使基体表面粗糙化 [17] [18],但脱碳过程中表面Co的转化及其对随后生长金刚石涂层的影响缺乏相关报道。本文表明采用管式炉及相应热处理气氛,可望将硬质合金表面的Co转化为Co3W,促进热丝化学气相沉积(HFCVD)金刚石涂层生长。

2. 实验材料与方法

采用市场购置的6wt.% Co硬质合金(Extramet)。管式炉热处理前,分别在去离子水和丙酮中对样品进行超声清洗15分钟。三种不同气氛处理条件,分别是A#1: 纯N2;A#2: N2/H2 = 50/3;A#3: N2/H2 = 50/6。固定处理温度700℃、时间4 h,总气压为常压。

沉积金刚石前,采用5wt.%的50 nm粒度的纳米金刚石粉乙醇悬浊液在预热处理样品表面超声植晶30 min。采用自主研发的热丝CVD设备沉积金刚石涂层,工艺参数包括:CH4/H2 = 1.5/100,沉积温度800℃,丝基距5 mm,热丝温度2200℃,气压4 KPa,沉积时间8 h。沉积后的A#1、A#2、A#3样品分别编号为Dia-A#1、Dia-A#2、Dia-A#3。

样品表面物相结构由Bruker XRD掠射模式进行表征(Cu Kα射线,工作电压是40 kV,扫描范围是(30˚~90˚)。表面/截面形貌、成分由蔡司Merlin扫描电子显微镜及Newton能谱仪进行表征。表面元素价态由Thermo Fisher Scientific K-Alpha XPS进行表征(Al Kα射线,束斑尺寸400 μm,步距0.050 eV)。涂层质量由HJY LabRAM Arami拉曼光谱仪进行表征(激光功率5 mV,波长532 nm,计数时间5 s)。涂层结合力由FT LC-200R型洛氏硬度计作定性表征(HRC压痕,加载时间是15 s)。

为获得Co化合物在样品表面的深度分布,进行了各种金相腐蚀以及背散射电镜照片表征,但效果不理想。故采用涂层球磨仪在表面磨出一定直径的球坑(由三角函数计算得球坑深度),将该球坑精磨抛光至不可观察为止,以便采用XPS表征不同深度的Co价态从而间接获得Co化合物的深度分布。

3. 实验结果与分析

3.1. 热处理后硬质合金表面的物相与组织

图1是A#1~A#3样品表面XRD谱图和形貌。热处理后A#1~A#3样品表面Co部分或全部转化为Co3W。图1(d)中存在许多富集O元素的白色板条晶体;图1(e)中存在许多弥散于深灰色区域的O与Co含量相当的白色小颗粒;图1(f)中深灰色区域富集Co,结合XRD判断为Co3W。

Figure 1. XRD patterns and surface morphology of samples (a)/(d): A#1; (b)/(e): A#2; (c)/(f): A#3

图1. 样品表面XRD谱图及表面形貌(a)/(d): A#1; (b)/(e): A#2; (c)/(f): A#3

图2是A#1和A#2样品轻微抛掉约1 μm后的表面形貌和XRD谱图。由于A#3样品表面无A#1和A#2样品表面白色物质的特征,故不作抛光及表征。图1中A#1和A#2样品表面的白色板条晶体和白色小颗粒基本消失。A#1样品表面的CoWO4物相强度明显降低,Co3W物相强度变化不明显,低倍选区EDS显示其表面O含量从原来的33.73at.%降至10.51at.%,表明热处理后A#1的白色板条是CoWO4晶体。A#2样品抛光前后其XRD谱图皆无氧化物的物相存在,低倍选区EDS显示表面O含量从原来的4.19at.%降为0,估计该白色小颗粒是含Co氧化物。EDS显示图2(a)和图2(c)标示的深灰色区域中Co:W的原子比接近3:1,结合XRD判断为Co3W。

对A#1~A#3样品不同深度处作XPS Co2p表征,并作高斯–洛伦兹分峰拟合 [19],计算0价态Co峰面积与所有不同价态Co峰面积之和的比值(记作k)。k值在A#1~A#3中随深度的变化如图3所示。在样品的1.5 μm深度处,A#1样品k值为5.7%,而A#2样品为12.1%。前者表面氧化严重,导致同等深度处,k值更小。当H2含量增加,A#3样品在1.5 μm深度处的k值为0,当深度达到3 μm时,k值为11.9%,说明A#3样品至少在1.5 μm以内不存在0价态的Co,即在一定的范围内,H2含量的增加有利于Co3W的转化。

Figure 2. Surface morphology and XRD patterns of samples after slightly polishing (a)/(b): A#1; (c)/(d): A#2

图2. 样品轻微抛光后的表面形貌和XRD谱图(a)/(b): A#1; (c)/(d): A#2

Figure 3. k Value in different samples vary with depth

图3. 不同样品中的k值随深度的变化

3.2. 硬质合金表面Co的化合物对金刚石涂层生长的影响

图4(a)、图4(c)、图4(d)分别是Dia-A#1、Dia-A#2、Dia-A#3的XRD谱图。图4(b)是Dia-A#1的表面形貌。A#1样品沉积后的表面覆盖了一层黑色的灰,未检测到金刚石物相。从图1~3结果看,A#1表面部分Co转化为Co3W,部分被氧化为CoWO4。后者在沉积气氛中容易被还原为Co,产生严重的石墨催化效应 [4] [5] [6]。沉积后A#1样品表面未检测到Co3W,但二元Co-W相图表明Co3W在1000℃以下是稳定存在的 [20],说明Co3W在含碳沉积氛围不稳定。图4(b) Dia-A#1样品中位置1和2的C含量高达88.86at.%和85.92at.%,很可能是植晶时表面吸附的残留纳米金刚石颗粒或者处于形核期的金刚石颗粒。因为XRD未检测到金刚石物相,表明Dia-A#1表面这样的颗粒含量极少。

图5是Dia-A#2和Dia-A#3样品的表面/截面形貌及EDS线扫描谱图。二者表面形貌差异较大,前者呈现为菜花状的纳米金刚石形貌,后者呈现为结晶度高且晶面发育完整的微米晶金刚石形貌,二者的涂层厚度都约为2.2 μm。

Figure 4. XRD patterns of Dia-A#1 (a), Dia-A#2 (c) and Dia-A#3 (d) and surface morphology of Dia-A#1 (b)

图4. Dia-A#1 (a)、Dia-A#2 (c)、Dia-A#3 (d)样品表面XRD谱图及Dia-A#1 (b)样品表面形貌

Figure 5. Surface/section morphology and EDS analysis (a)-(c): Dia-A#2; (d)-(f): Dia-A#3

图5. 表面/截面形貌和EDS分析(a)~(c): Dia-A#2; (d)~(f): Dia-A#3

图5(b)是Dia-A#2的截面形貌图,涂层与基体分离可能是产生图5(c)的EDS曲线分为I、II区域的原因,I区中所有元素信号均为0。III区域左侧出现Co信号,该信号可能:1) Co扩散进入碳涂层,同时导致石墨催化效应 [4] [5] [6];2) 涂层分离时黏在基体的Co相。因为靠近界面层附近存在Co,使III区碳含量出现分峰现象。Dia-A#2涂层的表面附近(III区域中)又有Co信号 [21],这一现象表明Co扩散在生长的碳涂层中的分布不均匀。正是因A#2样品表面部分Co转变为在沉积还原气氛中不稳定的氧化物,在热丝沉积过程中容易被还原为0价态Co,增强了金刚石涂层生长中的石墨化。

图5(f)是Dia-A#3涂层的EDS成分检测结果,可以认为碳涂层中Co信号比Dia-A#2弱很多;或者合理假定Dia-A#3涂层中无Co。这说明一定深度的Co几乎完全转变为Co3W化合物后,能有效地起到阻挡Co扩散的作用。该化合物在沉积过程中虽然会分解,但该分解反应未造成明显的石墨化。其中深入机理有待进一步分析。

图6是Dia-A#2和Dia-A#3样品经高斯分峰拟合后的Raman光谱,拟合所得数据如表1所示。Dia-A#2、Dia-A#3样品的金刚石峰位分别是1337.5 cm1和1335.1 cm1,均表现为残余压应力,且后者小于前者 [22]。1580 cm1及1350 cm1附近的峰位对应以sp2键结合的石墨相;1140 cm1和1480 cm1附近的峰位对应反式聚乙炔(TAP) [13] [23] [24]。Dia-A#3样品的金刚石峰FWHM小于Dia-A#2样品,结晶度高,与其表面形貌结果一致 [25]。Q因子(Q = I(T2g)/[I(D) + I(G)])能反映涂层中金刚石物相纯度 [14],表明Dia-A2涂层中石墨成分相对较高,这与表层钴氧化物转变为0价态Co、促进石墨催化有关 [4] [6];表面呈现出菜花状的纳米金刚石涂层形貌 [26] [27]。而A#3样品表面一定深度Co几乎完全转化为Co3W化合物,该化合物虽然在HFCVD工况下不稳定,但可有效抑制表面Co向涂层的扩散及涂层的石墨化,获得晶面发育完整的微米晶金刚石形貌。

Figure 6. Raman spectrum (a): Dia-A#2; (b): Dia-A#3

图6. Raman光谱(a): Dia-A#2; (b): Dia-A#3

Table 1. The result of peak fitting of Raman spectrum (a): Dia-A#2; (b): Dia-A#3

表1. 拉曼光谱的分峰拟合结果(a): Dia-A#2; (b): Dia-A#3

图7是Dia-A#2、Dia-A#3的HRC压痕形貌。与Dia-A#3相比,Dia-A#2涂层残余应力较大、涂层中石墨含量也较高,导致其涂层结合力较弱,在压痕附近出现涂层的大片崩落,而Dia-A#3只是出现了少许的涂层崩落。Dia-A#3金刚石涂层具有更好的结合力。

Figure 7. HRC indentation on the surface (a): Dia-A#2; (b): Dia-A#3

图7. 表面HRC压痕(a): Dia-A#2; (b): Dia-A#3

4. 结论

热处理将硬质合金表层Co几乎转化为多种化合物相。当气氛中H2含量小于6%时,表层转化相为复相,即Co3W及CoWO4;当气氛中N2/H2 = 50/6时,可使约1.5 μm内亚表层的Co几乎全部转化为Co3W。尤其当亚表层的Co仅以Co3W存在时,可以在热丝化学气相沉积金刚石时起到阻挡基体Co的向外扩散,有效地抑制涂层生长时的石墨化效应。

基金项目

本文得到广州科技计划项目(编号:201902010018、201807010091)及广东省科技计划项目(编号:2015B090923006)的支持。

NOTES

*通讯作者。

文章引用: 曾己伟 , 李伟秋 , 彭继华 (2020) 热处理对硬质合金表面Co转变及生长金刚石涂层的影响。 材料科学, 10, 562-569. doi: 10.12677/MS.2020.107068

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