金属学报, 2020, 56(11): 1521-1529 DOI: 10.11900/0412.1961.2020.00087

Ti-B-N纳米复合涂层的设计、制备及性能

刘艳梅1, 王铁钢,1, 郭玉垚1, 柯培玲2, 蒙德强1, 张纪福1

1 天津职业技术师范大学天津市高速切削与精密加工重点实验室 天津 300222

2 中国科学院宁波材料技术与工程研究所中国科学院海洋新材料与应用技术重点实验室 宁波 315201

Design, Preparation and Properties of Ti-B-N Nanocomposite Coatings

LIU Yanmei1, WANG Tiegang,1, GUO Yuyao1, KE Peiling2, MENG Deqiang1, ZHANG Jifu1

1 Tianjin Key Laboratory of High Speed Cutting and Precision Manufacturing, Tianjin University of Technology and Education, Tianjin 300222, China

2 Key Laboratory of Marine Materials and Related Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China

通讯作者: 王铁钢,tgwang@tute.edu.cn,主要从事硬质薄膜与刀具涂层技术研究

责任编辑: 肖素红

收稿日期: 2020-03-19   修回日期: 2020-05-22   网络出版日期: 2020-11-11

基金资助: 国家自然科学基金项目.  51301181
国家自然科学基金项目.  51875555
天津市科技重大专项项目.  18ZXJMTG00050
天津市自然科学基金项目.  19JCYBJC17100

Corresponding authors: WANG Tiegang, professor, Tel: (022)88181083, E-mail:tgwang@tute.edu.cn

Received: 2020-03-19   Revised: 2020-05-22   Online: 2020-11-11

Fund supported: National Natural Science Foundation of China.  51301181
National Natural Science Foundation of China.  51875555
Tianjin Science and Technology Major Project.  18ZXJMTG00050
Tianjin Natural Science Foundation.  19JCYBJC17100

作者简介 About authors

刘艳梅,女,1976年生,硕士

摘要

利用脉冲直流磁控溅射技术研制Ti-B-N涂层,通过降低反应气体N2流量,减少涂层中a-BN (a代表非晶)软质相的含量,增大TiB2靶溅射功率,提高硬质相TiB2的含量,形成nc-(Ti2N, TiB2)/a-BN (nc代表纳米晶)纳米复合结构,实现涂层增韧和强化。系统研究了TiB2靶溅射功率对Ti-B-N涂层成分、微观结构和性能的影响,利用EDS、HRTEM、SEM、XRD、纳米压痕仪和划痕测试仪对涂层进行表征和测试,利用球-盘式摩擦磨损试验机测试涂层摩擦学性能。结果表明,随着TiB2靶溅射功率增加,Ti-B-N涂层结构逐渐由nc-Ti2N/a-BN演变成hcp-TiB2/a-BN;Ti-B-N涂层的纳米硬度也逐渐增加,当TiB2靶溅射功率为2.4 kW时,涂层硬度最高,约为33.8 GPa;此时Ti-B-N涂层的摩擦系数和磨损率也最低,分别为0.55和2.1×10-4 μm3/(N·μm),涂层耐磨性能最佳。

关键词: 涂层强化 ; 磁控溅射 ; Ti-B-N涂层 ; 靶材溅射功率 ; 纳米硬度

Abstract

TiB2 coating comprises a large number of ionic and covalent bonds, conferring it with excellent properties such as high melting point, high hardness, and good oxidation and corrosion resistances. However, its application to cutting tool surfaces is limited due to high brittleness. When doped with N atoms, TiB2 coating forms a nanocomposite structure with improved toughness. However, the hardness of the resulting coating is significantly impaired by the abundant amorphous BN (a-BN) phase. The addition of metal ions and reactive N2 increases the proportion of hard nitrides and improves the coating hardness. However, the addition of N2 increases the amount of soft a-BN phase, which largely negates the strengthening effect. To further improve the mechanical properties of Ti-B-N coating, a series of Ti-B-N coatings were prepared by pulsed direct current magnetron sputtering in this work. The content of soft-phase a-BN in the coating was reduced by decreasing the flow of reactive gas N2. Meanwhile, the amount of hard TiB2 phase was increased by increasing the sputtering power of the TiB2 target. Consequently, a noncrystalline (nc)-(Ti2N, TiB2)/a-BN nanocomposite coating with significantly improved toughness and strength was formed. The influence of TiB2 target sputtering power on the composition, microstructure, and mechanical and tribological properties of the Ti-B-N coatings were systematically investigated by EDS, TEM, SEM, XRD, and nano-indentation, scratch, and ball-on-disk tribological testings. As the sputtering power of the TiB2 target increased, the microstructure of Ti-B-N coatings gradually evolved from nc-Ti2N/a-BN to hexagonal-close-packed TiB2/a-BN, and the nanohardness also increased gradually. The particle size on the coating surface was significantly increased, and all Ti-B-N coatings were uniform and compact without pinholes and other defects. The coating with highest hardness of about 33.8 GPa was achieved under a sputtering power of 2.4 kW at the TiB2 target. This coating also exhibited the lowest friction coefficient (0.55), lowest wear rate (2.1×10-4 μm3/(N·μm)), and best wear resistance.

Keywords: coating strengthening ; magnetron sputtering ; Ti-B-N coating ; target sputtering power ; nano hardness

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本文引用格式

刘艳梅, 王铁钢, 郭玉垚, 柯培玲, 蒙德强, 张纪福. Ti-B-N纳米复合涂层的设计、制备及性能. 金属学报[J], 2020, 56(11): 1521-1529 DOI:10.11900/0412.1961.2020.00087

LIU Yanmei, WANG Tiegang, GUO Yuyao, KE Peiling, MENG Deqiang, ZHANG Jifu. Design, Preparation and Properties of Ti-B-N Nanocomposite Coatings. Acta Metallurgica Sinica[J], 2020, 56(11): 1521-1529 DOI:10.11900/0412.1961.2020.00087

TiB2作为一种六方结构的陶瓷化合物,具有高熔点、超耐磨、良好的导电性和化学稳定性,已成为一种极具前景的新型材料[1~4],而TiB2涂层有望用于切削刀具及耐磨零部件的表面防护[5]。常见的TiB2涂层制备方法包括磁控溅射、电子束蒸发、离子束溅射等,Sanchez等[6]采用脉冲直流磁控溅射技术,在不同基体偏压下制备TiB2涂层,阐述了TiB2涂层的生长机理及内应力变化规律。Wang等[7]采用离子束溅射技术分别在50和480 ℃沉积TiB2涂层,发现TiB2涂层的晶格常数与沉积温度呈反比,当沉积温度为480 ℃时,制备的TiB2涂层硬度高达48 GPa。

尽管二硼化物涂层具有高硬度、高耐热能力等优点,但其脆性大,容易开裂甚至脱落,无法服役高强度冲击载荷工况,若向涂层中掺杂C、N等元素可形成蜂窝状纳米复合结构,即非晶原子层包覆纳米尺寸晶粒的三维结构,大量的两相界面可增加微裂纹扩展和位错运动的阻力,实现涂层增韧和强化[8],提升涂层的力学性能、摩擦磨损性能和抗氧化能力[9~11]。Neidhardt等[12]利用弧蒸发技术向Ti-B涂层中掺杂N元素,在高含量N2条件下生成nc-TiN/a-BN结构(nc代表纳米晶,a代表非晶),涂层摩擦系数为0.7~0.8。本课题组前期研究[13]发现,反应沉积Zr-B-N涂层时即使通入极少量N2,也会形成大量a-BN相,而显著降低涂层硬度。因为BN的形成能(-250.3 kJ/mol)远低于金属氮化物的形成能,掺杂的N元素优先与B反应生成软质BN非晶相。为提高涂层硬度,通常采用正向设计方法,即通过补充金属离子和通入大量反应气体(N2)来增加涂层中氮化物硬质相的数量,但同时也增大了涂层中软质非晶相的含量,强化效果不明显。另外,增加靶材溅射功率不仅改变涂层成分,而且增大了溅射粒子的能量,获得的涂层结构更加致密,进而改善涂层性能。喻利花等[14]以不同B靶功率制备系列Zr-B-N涂层,发现随着B靶溅射功率增加,涂层中fcc-ZrBN结构逐渐分解为hcp-ZrB2结构,晶粒尺寸逐渐减小,摩擦系数略有降低。左伟峰等[15]报道了Al靶溅射功率对TiAlN涂层的影响,发现Al靶溅射功率越大涂层组织愈致密,膜基结合力越强。

为进一步提升Ti-B-N涂层的力学性能,若在反应溅射时降低反应气体N2流量,减少涂层中a-BN软质相的含量,增大TiB2靶溅射功率,提高TiB2硬质相的含量,通过形成nc-(Ti2N, TiB2)/a-BN纳米复合结构,有望实现涂层增韧和强化。目前,关于调节TiB2靶溅射功率制备Ti-B-N涂层的研究仍鲜有报道,本工作采用脉冲直流磁控溅射技术研制Ti-B-N涂层,通过改变TiB2靶溅射功率调控涂层成分、结构和性能,系统研究了TiB2靶材溅射功率对涂层微结构和性能的影响机理,为硼氮化物涂层的工程化应用提供理论依据。

1 实验方法

1.1 涂层制备

采用脉冲直流磁控溅射技术在单晶Si (100)片和镜面抛光的SUS304不锈钢、YT硬质合金基体表面沉积Ti-B-N涂层,阴极连接TiB2靶材(纯度99.9%),靶基距为80 mm。将基体放置于酒精(纯度99.5%)中超声清洗20 min,然后使用N2 (纯度99.999%)吹干,最后固定于镀膜室内转架上。在镀膜之前,通入Ar (纯度99.999%)并控制节流阀使工作压强保持在1.4 Pa,施加-800 V偏压,辉光清洗基片10 min。再将偏压降至-50 V,通入反应气体N2,保持工作压强为0.6 Pa,开启TiB2靶电源用于沉积Ti-B-N涂层,所有涂层沉积时间均为360 min。本工作选择TiB2靶溅射功率区间为0.8~2.4 kW,前期实验[13]发现当硼化物靶材溅射功率低于0.8 kW时,通入反应气体N2后,靶表面出现明显钝化、不易起辉,涂层沉积速率显著降低,且性能较差;当硼化物靶溅射功率大于2.4 kW时,所制备的涂层硬度高、脆性大,承载后易开裂,同时镀膜过程中靶材周围热量无法被(冷却水)及时带走,易发生靶材变形甚至烧穿。具体工艺参数详见表1

表1   不同TiB2靶溅射功率制备Ti-B-N涂层的工艺参数

Table 1  Deposition parameters of the Ti-B-N coatings prepared by different sputtering powers of the TiB2 target

ParameterValueUnit
Base pressure3.0×10-3Pa
Working pressure0.6Pa
Pulsed DC sputtering power (TiB2 target)0.8, 1.2, 1.6, 2.0, 2.4kW
Bias voltage-50V
Ar∶N2 flow ratio96∶4
Deposition temperature300
Substrate rotation speed30r·min-1
Distance between target and substrate80mm
Deposition time360min

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1.2 成分分析和结构表征

利用Nano430扫描电子显微镜(SEM)观察不同Ti-B-N涂层的表面和截面形貌;利用EPMA-1600电子探针分析仪(EPMA)定量分析涂层化学成分;利用D8 ADVANCE X射线衍射仪(XRD)分析涂层的物相组成,选用CuKα射线(波长λ=0.154056 nm)进行辐射,衍射角2θ扫描范围为22.5°~80°,扫描步长0.01°。利用聚焦离子束(FIB)制备涂层截面透射电镜样品,使用Tecnai G2 TF20 UT高分辨透射电子显微镜(HRTEM)结合选区电子衍射(SAED)谱识别涂层截面的微观结构信息。上述实验,涂层样品的基体均为Si片。

1.3 性能测试

利用TTX NHT-3纳米压痕仪测试涂层的纳米硬度,基体为单晶Si片,压痕测试选用Berkovich金刚石针尖(弹性模量1140 GPa,Poisson比0.07,针尖直径200 μm),保压时间为10 s,法向载荷5 mN,所有压痕深度均不超过涂层厚度的10%[16],每个涂层样品选取不同位置测试20次,取平均值作为硬度测试结果。采用Anton Paar RST-3划痕仪测量涂层与基体的结合强度,基体为SUS304不锈钢,测试时法向载荷从零逐渐增加到100 N,加载速率为1 N/s,划行速率0.1 mm/s,划痕长度10 mm,借助KEYENCE VHX-1000C光学显微镜(OM)识别涂层开始剥落位置、确定临界载荷,每个涂层样品测试5次取平均值。采用Anton Paar THT球-盘式摩擦磨损试验机测试Ti-B-N涂层的摩擦系数,基体为YT硬质合金,摩擦副选用直径为6 mm的Al2O3球,摩擦实验在室温下进行(相对湿度30%~40%),磨痕直径8 mm,滑动速率0.1 m/s,法向载荷5 N,每个样品旋转3000 r,滑动距离75.4 m,取稳定摩擦阶段的数据计算平均摩擦系数,利用OM观察磨痕形貌。利用D500 Stylus Profilometer轮廓仪测量涂层磨痕横截面积和深度,根据公式W=A/(F·n) (其中,A为磨痕横截面积,F为载荷,n为摩擦圈数)计算涂层磨损率。

2 实验结果与分析

2.1 涂层成分与相结构

图1为不同TiB2靶溅射功率制备Ti-B-N涂层的化学成分。可见,随着TiB2靶溅射功率的增加,涂层中Ti含量由6.0% (原子分数,下同)线性增长至29.2%。因为随着靶溅射功率的增加,单位时间内从靶表面溅射出的Ti、B粒子和TiB2分子数量增多,导致涂层中Ti含量呈增长趋势。而N含量从20.1%下降到4.4%,B含量也从74.0%小幅降至66.5%。这是因为BN的结合能要低于Ti2N和TiB2,在涂层中很容易形成BN相,而低分子量的BN颗粒动能较小,单位时间内沉积到基体表面的粒子数量较少[17]。另外,溅射功率增加使等离子体中Ti、B粒子增多,导致反应气氛中N含量降低,也将引起涂层中N含量降低。

图1

图1   不同TiB2靶溅射功率制备Ti-B-N涂层的化学成分

Fig.1   Chemical compositions of the Ti-B-N coating prepared by different sputtering powers of the TiB2 target


图2为不同TiB2靶溅射功率制备的Ti-B-N涂层的XRD谱。可见,当TiB2靶溅射功率为0.8 kW时,涂层中只出现了(101)取向的Ti2N相衍射峰;当TiB2靶溅射功率逐渐增加至1.6 kW时,Ti2N相衍射峰强度逐渐减弱,并最终消失[18,19]。根据图1可知,此时涂层中的N含量较低,说明镀膜室中的N与B优先反应生成BN相[20],没有剩余N与Ti反应生成Ti2N相,故Ti2N相衍射峰逐渐消失。同时也检测到沿(101¯0)和(101¯1)晶面生长的TiB2陶瓷相,表明TiB2靶溅射功率增加会促进TiB2相结晶[21]。继续增加靶溅射功率至2.4 kW,分别检测到沿(0001)、(101¯0)和(101¯1)晶面生长的3种TiB2相衍射峰,且TiB2相沿(0001)晶面择优生长[22]。原因在于反应气氛中N含量较少导致金属氮化物产量不足,相应地涂层中TiB2相结晶度增加。此外,所有涂层均未检测到BN相衍射峰,它可能以非晶体形式存在,这也是所有涂层衍射峰发生宽化的重要原因。

图2

图2   不同TiB2靶溅射功率制备Ti-B-N涂层的XRD谱

Fig.2   XRD spectra of the Ti-B-N coatings prepared by different sputtering powers of the TiB2 target


2.2 微观结构与沉积速率

图3为不同TiB2靶溅射功率制备的Ti-B-N涂层截面和表面形貌的SEM像。由图3a~e可见,所有涂层结构均匀致密,无针孔等缺陷,且与基体结合良好,随着TiB2靶溅射功率增加,涂层厚度逐渐增大。在图3a~c中涂层截面形貌无明显特征,存在的纹理是由于切断试样时受力不均匀所致。由于靶材溅射功率较低时,溅射产物数量少,掺杂N元素后形成大量的非晶BN相,可抑制晶粒生长,起到细化晶粒作用[23]。当靶溅射功率增至2.4 kW时,涂层中出现明显的柱状晶结构,此时溅射产物增多,N元素已完全反应,过量的TiB2溅射颗粒沉积到基体表面,形核并结晶,生长成柱状晶结构。从图3f~j可以看出,当靶材溅射功率较小时,溅射产物少,涂层表面细腻光滑,未出现大颗粒和较大金属液滴。随着靶材溅射功率增加,涂层表面出现大量微小凸起,且边界清晰,分别对应于柱状晶的顶部。一方面,凸起的产生与沉积粒子在基体表面的迁移率密切相关[24],靶材溅射功率增加,加剧了溅射粒子在基体表面的迁移和扩散,晶粒开始生长变大,逐渐形成球状颗粒。另一方面,结合图2可知,尽管所有Ti-B-N涂层中非晶BN相可以起到晶粒细化的作用,但随着TiB2靶材溅射功率增加,涂层中TiB2相含量增多提高了涂层的结晶度,而非晶BN相数量趋于稳定[25],故涂层表面凸起尺寸变大。

图3

图3   不同TiB2靶溅射功率制备Ti-B-N涂层截面和表面形貌的SEM像

Fig.3   Cross-sectional (a~e) and surface (f~j) SEM images of the Ti-B-N coatings prepared by different sputtering powers of the TiB2 target

(a, f) 0.8 kW (b, g) 1.2 kW (c, h) 1.6 kW (d, i) 2.0 kW (e, j) 2.4 kW


当TiB2靶溅射功率为2.4 kW时制备的Ti-B-N涂层结晶度最高,利用HRTEM进一步观察其显微结构,如图4所示。可以看出,尺寸为5~12 nm的TiB2晶粒(白色圆环内) 镶嵌在非晶组织中,部分晶格条纹排列呈不连续状,表明Ti-B-N涂层中晶粒尺寸保持在纳米级别[26]。同时,SAED花样(图4中插图)显示为几乎连续状衍射环和云雾状衍射花样,经EDS分析,非晶组织富含B元素,研究[27]表明BN相在Ti-B-N涂层内以非晶形式存在,故此Ti-B-N涂层具有典型的纳米复合结构,并随着TiB2靶溅射功率增加,涂层逐渐由nc-Ti2N/a-BN演变为hcp-TiB2/a-BN结构。

图4

图4   TiB2靶溅射功率为2.4 kW时制备Ti-B-N涂层的HRTEM像和相应的SAED花样

Fig.4   HRTEM image and SAED pattern (inset) for the Ti-B-N coating prepared under a sputtering power of 2.4 kW at the TiB2 target (The zones marked with ellipse represent the TiB2 nanocrystals)


图5所示为不同TiB2靶溅射功率制备的Ti-B-N涂层的沉积速率。由图可知,随着TiB2靶溅射功率增加,Ti-B-N涂层的沉积速率由2.8 nm/min近似线性上升至7.1 nm/min。根据Depla等[28]报道,即使镀膜室内通入少量N2也会引起“靶钝化”,由于部分氮化物吸附于靶表面,抑制或消减了溅射粒子数量,从而降低涂层沉积速率。依据图2可知,当TiB2靶溅射功率为2.4 kW时,涂层中未检测到Ti2N衍射峰,间接表明靶材溅射功率的增加将减轻“靶钝化”。因为增加靶材溅射功率会增大靶表面电流密度,从而提高溅射粒子数量和动能,导致涂层沉积速率上升。

图5

图5   不同TiB2靶溅射功率制备Ti-B-N涂层的沉积速率

Fig.5   Deposition rates of the Ti-B-N coatings prepared by different sputtering powers of the TiB2 target


2.3 涂层的力学性能

硬度是衡量涂层力学性能的重要指标,并受涂层结构、内应力和晶粒尺寸等因素影响[29]图6为不同TiB2靶溅射功率制备的Ti-B-N涂层的纳米硬度。由图可见,随着TiB2靶溅射功率增加,涂层硬度总体呈上升趋势。当TiB2靶溅射功率为0.8 kW时,硬度较低,约为18.1 GPa,原因在于硼化物涂层的硬度受B—B共价键影响较大;结合图2可知,此时涂层结晶度偏低,且软质BN非晶相含量较高,对涂层硬度产生消极影响。进一步增加靶溅射功率至2.4 kW时,涂层硬度高达33.8 GPa。一方面,TiB2相沿(0001)晶面择优生长,有助于提高涂层硬度;另一方面,纳米晶TiB2相和非晶BN相形成了纳米复合结构,两相界面之间存在较大的内聚能,可阻碍晶粒滑移和位错运动,实现界面强化[30]

图6

图6   不同TiB2靶溅射功率制备Ti-B-N涂层的纳米硬度

Fig.6   Nanohardnesses of the Ti-B-N coatings prepared by different sputtering powers of the TiB2 target


利用划痕法来判定涂层的膜/基结合力。由于界面性质的差异,涂层和基体的硬度、结构和厚度都影响着临界载荷。图7显示了Ti-B-N涂层的临界载荷随TiB2靶溅射功率的变化趋势。可见,涂层临界载荷在61~86 N范围内变化,并随着TiB2靶溅射功率增加呈现先上升后下降趋势。当靶材溅射功率在0.8~1.6 kW范围变化时,涂层的临界载荷均较高,由于此时涂层内非晶相含量高而晶粒尺寸较小,在复杂应力作用下晶粒越小越容易产生微滑移或倾斜,不利于裂纹萌生、扩展和薄膜剥落,有助于增强膜/基结合力[31]。进一步增加TiB2靶溅射功率至2.4 kW,临界载荷近似线性降至最小值约61 N,靶材溅射功率增加会加剧溅射粒子对基体的轰击效应,使基体的温度升高,热应力的引入在一定程度上可削弱膜/基结合强度[32]。另根据图3可知,此时涂层表面颗粒较大,对应截面已出现柱状晶,在剪切应力作用下会加剧晶粒间滑移、易引起穿晶断裂,裂纹进一步扩展传播导致涂层与基体剥离。

图7

图7   不同TiB2靶溅射功率制备的Ti-B-N涂层的临界载荷

Fig.7   Critical loads of the Ti-B-N coatings prepared by different sputtering powers of the TiB2 target


2.4 涂层的摩擦学性能

图8为不同TiB2靶溅射功率制备的Ti-B-N涂层的平均摩擦系数。可见,随着TiB2靶溅射功率增加,Ti-B-N涂层的摩擦系数呈先降低后增大再下降趋势。当靶材溅射功率较低时,涂层中形成了大量的软质非晶BN相,在摩擦过程中逐渐发生剥落并转移至摩擦界面,具有一定的润滑功能[31]。当溅射功率增至1.2 kW时,涂层内硬质相增多并形成了nc-(Ti2N, TiB2)/a-BN纳米复合结构,涂层硬度和韧性均得到提升,摩擦过程不易出现碎屑剥落,可显著减小对摩副界面摩擦,涂层摩擦系数降至0.58。进一步增加TiB2靶溅射功率至2.0 kW,摩擦系数再次增大。结合物相和硬度分析可知,此时涂层内富含脆性大、硬度高的TiB2[33],摩擦过程中涂层碎片易发生剥落并逐渐转移至摩擦界面,导致摩擦系数升高。当TiB2靶溅射功率为2.4 kW时,根据图6可知,此时涂层硬度最高,但摩擦系数骤降至最低0.55。研究[34,35]表明,涂层硬度越高对外载荷抵抗能力越强,在稳定摩擦阶段对摩副间接触面积越小,有助于减小摩擦,这也遵循Archard理论[36,37]。可见,Ti-B-N涂层成分、物相组成、硬度等因素均影响其摩擦系数。

图8

图8   不同TiB2靶溅射功率制备的Ti-B-N涂层的平均摩擦系数

Fig.8   Average friction coefficients of the Ti-B-N coatings prepared by different sputtering powers of the TiB2 target


图9为不同TiB2靶溅射功率沉积的Ti-B-N涂层的磨痕形貌。可见,当TiB2靶溅射功率为0.8 kW时,涂层局部有脱落,并已被磨穿。结合图3,此时涂层内含有较多软质非晶BN相,涂层硬度低导致了耐磨性变差。在图9b~e中,所有涂层均未被磨穿,但图9c中的磨痕内堆积了大量磨屑,图9d的磨痕宽度最大。当TiB2靶溅射功率为2.0 kW时,Ti-B-N涂层硬度较高,摩擦过程中加剧了Al2O3对磨球的磨损,使摩擦接触界面增大,摩擦系数也增加[38,39]。继续增加TiB2靶溅射功率至2.4 kW,涂层磨痕最窄且表面出现微犁沟,磨损机制以磨粒磨损为主[40],归因于较硬的磨屑参与摩擦所致。

图9

图9   不同TiB2靶溅射功率沉积的Ti-B-N涂层磨痕形貌的OM像

Fig.9   OM images of worn scar of the Ti-B-N coatings prepared by different sputtering powers of the TiB2 target

(a) 0.8 kW (b) 1.2 kW (c) 1.6 kW (d) 2.0 kW (e) 2.4 kW


图10为不同TiB2靶溅射功率沉积的Ti-B-N涂层的磨损率。可见,随着TiB2靶溅射功率增加,涂层的磨损率先增大后减小。当TiB2靶溅射功率为0.8 kW时,涂层已被磨穿无法准确计算磨损率。当靶材溅射功率为2.0 kW时,磨损率较高,约为4.8×10-3 μm3/(N·μm),此时摩擦系数也最大,摩擦过程中较大的摩擦力和温升加剧了涂层的磨损,导致磨损率增加,这与磨痕形貌结果相一致。当TiB2靶溅射功率增至2.4 kW时,磨损率降至最低2.1×10-4 μm3/(N·μm),表现出优异的耐磨性能,此时涂层内硬质相增多,纳米复合结构中大量的两相界面具有增韧和强化作用。另外,涂层硬度高也是其耐磨损的重要原因[41]

图10

图10   不同TiB2靶溅射功率沉积的Ti-B-N涂层的磨损率

Fig.10   Wear rates of the Ti-B-N coatings prepared by different sputtering powers of the TiB2 target


3 结论

(1) 利用脉冲直流磁控溅射技术研制Ti-B-N纳米复合涂层,通过改变TiB2靶溅射功率调控涂层成分、结构和性能。随着TiB2靶溅射功率增加,涂层结构逐渐由nc-Ti2N/a-BN演变成hcp-TiB2/a-BN,最终以沿(0001)晶面择优生长的TiB2相为主,涂层结晶度逐渐升高。

(2) 随着TiB2靶溅射功率增加,涂层沉积速率近似线性上升;划痕法测得的临界载荷先上升后降低;涂层纳米硬度逐渐增加。当TiB2靶溅射功率为2.4 kW时,涂层硬质相含量最多,纳米硬度高达33.8 GPa,具有良好的力学性能。

(3) 当TiB2靶溅射功率为2.4 kW时,涂层的摩擦系数和磨损率均最低,分别为0.55和2.1×10-4 μm3/(N·μm),涂层磨痕窄且浅,磨损机理以磨粒磨损为主,涂层具有良好的减摩和耐磨性能。

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