Al/Ti摩尔比对(CrTiAl)N硬质膜相结构和硬度的影响
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Effect of Al/Ti Molar Ratio on Phase Structure and Hardness of (CrTiAl)N Films
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通讯作者: 张钧,教授,zhjun88@126.com,研究方向为材料表面改性
收稿日期: 2021-04-29 修回日期: 2021-07-13
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Corresponding authors: ZHANG Jun, Tel:
Received: 2021-04-29 Revised: 2021-07-13
作者简介 About authors
张钧,男,1966年生,博士
采用多弧离子镀技术,设计沉积工艺和调整阴极弧源靶组合以及对应的弧源电流,制备出以CrN为基形貌和厚度相同、Al/Ti摩尔比不同的系列(CrTiAl)N硬质膜。测试膜的成分、组织形貌、相组成和表面硬度,研究了 Al/Ti摩尔比对其相结构和硬度的影响。结果表明:不同Al/Ti摩尔比的(CrTiAl)N膜其相组成相同,都呈现(200)和(111)晶面择优生长。随着膜层的Al/Ti摩尔比从0.38提高到0.85,其硬度表现出规律性的变化。Al/Ti摩尔比为0.49的(CrTiAl)N膜硬度最高(达到HV4200),Al/Ti摩尔比为0.85时硬度降到HV2600。在膜层组织形貌、膜层厚度以及CrN含量基本不变的条件下,Al/Ti摩尔比直接影响膜层的硬度并呈现出非单调关系。在较大的Al/Ti摩尔比范围内(CrTiAl)N膜层的相结构不变,均为置换式面心立方固溶体。优化Al/Ti摩尔比,可使(CrTiAl)N膜的硬度显著高于(CrTi)N膜而具有超硬性。
关键词:
A series of (CrTiAl)N hard films with the same morphology and thickness but different Al/Ti molar ratio were prepared via multi-arc ion plating technology with desired processing parameters while varying the combination mode of cathode arc source and target. The chemical composition, microstructure, phase constituent and surface hardness of the films were characterized. Meanwhile the effect of Al/Ti molar ratio on the phase structure and hardness of the films were investigated. The results show that:the (CrTiAl)N films with different Al/Ti molar ratios present the same phase constituents with preferred growth orientations (200) and (111). With the increase of Al/Ti molar ratio from 0.38 to 0.85, the hardness of the film shows a regular change. The hardness of (CrTiAl)N films with Al/Ti molar ratio of 0.49 is the highest, reaching HV4200, however when the Al/Ti molar ratio is 0.85, the hardness decreases to HV2600. The Al/Ti molar ratio has a direct effect on the hardness of the film and shows a non-monotonic relationship when the microstructure, thickness and CrN content of the films are basically unchanged. In the larger Al/Ti molar ratio range, the phase structure of (CrTiAl)N films is unchanged and all of them are displacement-centered cubic solid solutions. The hardness of (CrTiAl)N films is significantly higher than that of (CrTiN) films and which can possess super hardness by optimizing the molar ratio of Al/Ti.
Keywords:
本文引用格式
张钧, 彭立静, 王宇, 王晓阳, 王楠, 王美涵.
ZHANG Jun, PENG Lijing, WANG Yu, WANG Xiaoyang, WANG Nan, WANG Meihan.
1 实验方法
使用多弧源电弧离子镀设备(MAD-4B型)在W18Cr4V高速钢基体上沉积(CrTiAl)N硬质膜。为了保证所沉积的(CrTiAl)N硬质膜具有可对比性,均采用双阴极弧源靶[11],分别为Ti-Cr合金靶(名义成分为Ti∶Cr=60%∶40%,原子分数)和Cr-Al合金靶(名义成分为Cr∶Al=60%∶40%,原子分数)。对2个弧源靶电流进行不同组合以实现膜层成分的改变(表1)。其它沉积条件,如气体流量(压强)、沉积时间、沉积温度,则相同,各镀膜试样分别标记为1#~4#。为了与(TiCr)N硬质膜比较,使用2个Ti-Cr合金靶(名义成分为Ti∶Cr=60%∶40%),沉积弧电流均为55 A,其它沉积条件同上,制备出的试样标记为0#。为了使膜层具有较高的附着强度,在氩气(3.0×10-1 Pa)气氛中用高负偏压(-320 V~-350 V)双靶弧光预轰击高速钢基片的方式清洗12 min,弧光轰击开始初始镀膜温度为180℃。然后,依次沉积合金过渡层(持续8 min、偏压降为-200 V)、低N2流量膜层(持续15 min、偏压-180 V)、正常N2流量膜层(持续37 min、偏压-180 V),如图1所示。镀膜结束时真空室的温度为200±10℃。
表1 (CrTiAl)N和(TiCr)N硬质膜的沉积工艺参数
Table 1
Film samples | Cathodic arc current/A | |
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TiCr | CrAl | |
1# | 60 | 48 |
2# | 55 | 50 |
3# | 50 | 55 |
4# | 48 | 60 |
0# | 55\55 | ~ |
图1
用S-4800Ⅱ型扫描电镜(能谱)观察(CrTiAl)N膜和(TiCr)N膜的表面形貌和断面形貌。用X’Pert PRO型X射线衍射仪分析膜层的相组成,辐射源为Cu靶(λ=0.154 nm),电流40 mA,电压40 kV,扫描范围为10°~90°,步长为0.05°。用402MVD型维氏硬度计测试高速钢基片表面膜层的硬度,同一视场分取三个位置,荷载为10 gf,保持时间为15 s。
2 结果和讨论
2.1 膜层表面和断面形貌
图2
图2
(CrTiAl)N膜和(TiCr)N膜的表面和断面形貌
Fig.2
Surface and fracture section morphology of the as-deposited (CrTiAl)N and (TiCr)N films
在本文的沉积实验中,设定两个阴极弧源靶的电流之和基本相同,为105~110 A,且其沉积时间均为8+15+37 min;这些条件,保证了沉积过程中氩气、氮气流量相同,使膜层沉积温度(200±10℃)、膜层N含量、膜层的微观组织形貌以及膜层厚度基本相同(试样0#膜层厚度稍微偏大)。因为膜层成分的差别主要来源于弧源靶成分的差别,膜层性能的差别主要决定于膜层元素(膜层成分)和相组成。而膜层的相组成又由膜层的成分变化所决定,以上条件保证了以膜层成分作为膜层性能变化的单一影响因素。
2.2 膜层表面成分分析
图3
图3
(CrTiAl)N膜的Al/Ti摩尔比以及对应的(归一化后)金属元素含量
Fig.3
Al/Ti molar ratio and the corresponding metal content of (CrTiAl)N films (normalized)
2.3 膜层的相结构
对膜层样品进行小角度XRD测试并使用JADE软件处理,得到图4a中的衍射谱。可以看出,所有膜层样品XRD衍射谱的衍射峰强度分布相似。结合CrN、TiN的标准衍射图谱(图4c和4d)确定出,(CrTiAl)N膜由有置换式固溶体型特征的面心立方(CrTiAl)N相和TiCr(Al)合金相组成,没有TiN相、CrN相以及AlN相。膜层表面的大颗粒来自于沉积过程中的阴极弧源靶表面金属熔滴,生成了由Cr、Ti(Al)元素组成的CrTi(Al)合金相[15,17]。分析各膜层的XRD图谱发现,随着Al/Ti摩尔比的增大衍射峰趋于向大角度偏移(图4b),表明Al原子部分替代Ti原子使晶格常数减小。采用织构系数计算公式计算出面心立方(CrTiAl)N相(200)、(111)晶面的衍射系数,其值均大于1,表明(CrTiAl)N膜层具有(200)、(111)晶面的双择优生长取向。
图4
图4
(CrTiAl)N膜的XRD衍射谱和CrN、TiN的标准衍射谱
Fig.4
XRD patterns of (CrTiAl)N films and the standard diffraction patterns of CrN and TiN films
同时,多弧离子镀TiAlN/TiN多层膜的研究表明,工艺参数(负偏压增大)的变化使择优取向从(200)晶面转变到(111)晶面[26];而对多弧离子镀TiAlN/CrN多层膜的研究则表明,虽然单层膜TiAlN和CrN均具有(200)晶面的择优生长取向,但是TiAlN/CrN多层膜的择优取向却是(111)晶面[27]。这些结果表明,沉积技术工艺的不同使制备出的Ti、Al、Cr多组元氮化物膜系中各个膜层的择优取向不同,却都容易形成固溶体式结构而不容易出现TiN、CrN、AlN等分离的第二相。在本文的研究中,由于前述沉积工艺基本相同,单一的成分变化并未引起相组成和晶体结构形式的变化,所有膜层均为置换式固溶体型特征的面心立方(CrTiAl)N相构成。这表明,这些膜层性能的变化源于成分变化的影响,而这种影响主要是晶格畸变引起的固溶强化。
根据(CrTiAl)N膜层XRD谱中特征衍射峰的位置,使用JADE软件可计算出面心立方固溶体型(CrTiAl)N相的晶格常数;将(CrTiAl)N膜当做TiN相、CrN相及AlN相形成的置换式固溶体处理,其各自比例以金属元素浓度归一化后的比例代替,根据VEGARD定律可计算出各组膜层试样面心立方固溶体结构的理想化晶格常数,如图5所示。可以看出,随着膜层Al/Ti摩尔比的增大晶格常数减小。其原因是,在CrN比例一定的条件下Al/Ti摩尔比的增大意味着Al含量的增大和Ti含量的降低,即点阵中的部分Ti原子被Al原子所代替,从而引起晶格收缩形变,使晶格常数减小。同时,与CrN、TixZryN、Ti1-xCrxN等氮化物膜的结果相似,实际测得的本文所有(CrTi)N、(CrTiAl)N膜层试样的晶格常数均高于根据VEGARD定律的理论计算值[22,25]。膜层的沉积过程为非平衡过程,膜层中的点缺陷和残余热应力使晶格常数增大[28]。与(CrTi)N膜层相比,(CrTiAl)N膜层的晶格畸变更为复杂,更容易使缺陷增加从而使晶格常数与VEGARD定律计算值的偏离更大。而随着Al含量的提高(Al/Ti摩尔比≤1),这个偏离随之变大。
图5
图5
(CrTiAl)N膜的实测晶格常数与根据VEGARD定律计算结果的对比
Fig.5
Comparison between the measured lattice constants of (CrTiAl)N films and the calculated from Vegard's law
2.4 膜层的显微硬度
所有膜层样品显微硬度的测试结果,如图6所示。可以看出,Ti58Cr42N膜层的硬度约为3500HV,Al原子部分替代Ti原子,生成了(CrTiAl)N固溶体相。Al/Ti摩尔比较小时,膜层的硬度没有大的提高;当Al/Ti摩尔比接近0.5时膜层硬度出现极大值,达到4200HV;而Al/Ti摩尔比进一步增大则膜层硬度降低,Al/Ti=0.85时的硬度甚至降为2600HV,明显低于Ti58Cr42N膜层的硬度。这表明,在CrN浓度不变的条件下,加入少量的Al原子并不能使(CrTi)N固溶体膜层发生进一步的固溶强化,只有当Al浓度提高到一定程度才能显著提高(CrTiAl)N固溶体相的硬度,而加入过量的Al原子反而使(CrTiAl)N固溶体相的硬度降低。
图6
图6
Al/Ti摩尔比对(CrTiAl)N膜硬度的影响
Fig.6
Effects of Al/Ti mole ratios on hardness of (CrTiAl)N films
在保持面心立方晶格结构的前提下,随着第二金属组元加入,两个金属组元成分趋近,Ti-Al-N、Ti-Zr-N、Ti-Nb-N、Cr-Al-N等三组元置换式固溶体氮化物硬质膜层的硬度增大,当金属组元比例约为1:1时达到极大值。其原因是,晶格畸变产生了固溶强化[7~9]。本文的研究实现了相近的沉积工艺、相同的膜层组织、厚度和相结构,各膜层的差别只是成分变化引起的固溶体中晶格畸变引起的。一方面,作为对照的面心立方固溶体Ti58Cr42N膜层,其晶格畸变有明显的强化作用,硬度接近于(TixCry)N固溶体膜层系中的硬度极大值;另一方面,Al原子的加入输出了(CrTiAl)N固溶体相,TiN、CrN、AlN晶格常数的差异使晶格畸变更为复杂。从膜层成分的角度来看,即使CrN的含量不变,Al/Ti摩尔比的增大和Al、Ti、Cr的含量趋近(图3a和b)仍然不能保证单调的晶格畸变加剧。这表明,(CrTiAl)N固溶体型膜层的硬度并不随着Al/Ti摩尔比的增大而单调变化。
3 结论
(1) 以Ti58Cr42N为基础,在确定Cr含量(在金属元素中的占比为42%)的条件下,在(CrTi)N中添加第三金属组元Al可形成(CrTiAl)N置换式固溶体膜层,不会形成CrN、TiN和AlN等杂相。
(2) 在Cr含量基本不变的条件下,单一改变Al/Ti摩尔比直接影响(CrTiAl)N膜层硬度,在硬度与Al/Ti摩尔比之间呈现非单调变化关系;与(CrTi)N膜相比,第三金属组元Al的添加并不能总是保证提高(CrTiAl)N置换式固溶体膜层的硬度。
(3) 在Cr含量基本不变的条件下,优化Al/Ti摩尔比可使(CrTiAl)N膜层的硬度明显高于(CrTi)N膜并达到超硬性。
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