1.College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China 2.Beijing Radiation Center, Beijing 100875, China 3.Science and Technology on Plasma Dynamics Laboratory, Air Force Engineering University, Xi’an 710038, China
Fund Project:Project supported by the Key Area R&D Program ofGuangdong Province, China(Grant No. 2019 B090909002), the National Science and Technology Major Projectof the Ministry of Science and Technology of China (Grant No. 2017-VII-0012-0107), and the National Defense Science and Technology Key Laboratory Fundof China (Grant No. 614220207011802)
Received Date:06 January 2020
Accepted Date:18 April 2020
Published Online:20 May 2020
Abstract:There are some high requirements for mechanical property to protective coatings of turbojet engine compressor blades as the appearance of extreme service conditions. The hard coating with high toughness, good adhesion, good wear resistance and excellent load carrying capacity is a potential coating for extreme service conditions in the future. Thick yet tough TiN hard coatings were successfully deposited on 304L stainless steel substrates by magnetic filtered cathodic vacuum arc technology. The morphology, structure and properties of the coatings were studied by SEM and XRD, etc.The results show that the continuous growth of TiN coatings attributed to periodic high energy ion bombardment which can suppress the large grain size and reduce the internal stress. The thickness of TiN coating can reach to 50 μm and the deposition rate was close to 0.2 μm/min. At the same time, the stable non stoichiometric TiN0.9 can be formed by controlling the constant N2 flow rate, which can improve the toughness of TiN coatings. All TiN ciatings belong to superhard coating and the max value of hardness and modulus of elasticity were 38.24 GPa and 386.53 GPa respectively. TiN coatings have good adhesion and excellent toughness.The highest $ H/E^{*} $ and $ H^3/E^{*2} $rate of TiN coating can reach to 0.0989 and 0.3742. Thick yet tough TiN hard coatings have excellent wear resistance with the lowest friction coefficient of 0.26. Keywords:compressor blades/ thick yet tough TiN coatings/ ion bombardment/ wear resistance
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2.1.试样制备
图1为磁过滤阴极弧等离子体沉积设备示意图. 磁过滤阴极弧等离子体设备主要由不锈钢真空室、样品台、缠绕铜线圈的磁过滤弯管、阳极筒和阴极靶组成. 机械泵与分子泵组成的抽真空系统可把不锈钢真空室气压抽至10–3 Pa量级. 样品台可被程序设置绕竖直轴公转和绕水平轴自转. 阴极靶在场致效应作用下离化成等离子体. 图 1 磁过滤阴极弧等离子体沉积设备示意图 Figure1. The schematic diagram of FCVAD system.
选用尺寸为20 mm × 20 mm × 0.5 mm的304L不锈钢作为基片. 不锈钢基片在放入真空室前, 用丙酮浸泡3 min后, 放入无水乙醇中超声清洗5 min. 清洗后的304L不锈钢基片安装在样品台中心位置, 距磁过滤弯管末端150 mm. 阴极靶材选用99.99%纯度的Ti金属靶, 靶直径为100 mm. 利用机械泵与分子泵进行抽真空, 当真空室气压为4 × 10–3 Pa时, 对基片表面进行溅射清洗, 分别设置基片偏压为–600, –400和–200 V, 不同偏压下的溅射清洗时间设置为2 min. 溅射清洗结束后, 保持偏压–200 V, 设置弧电流为90 mA, 在基片表面沉积一层纯金属Ti, 作为过渡层, 其厚度不超过总膜厚的10%. 图2为制备超厚TiN涂层的工艺示意图. 基底表面沉积Ti过渡层后, 以15 sccm (1 sccm = 1 ml/min)的流量通入N2, N2分子被Ti等离子体离化, 与Ti等离子体在基片上共同形成TiN涂层. 通入N2后的真空室气压为1.1 × 10–2 Pa. TiN涂层沉积过程中, 每隔1 h对TiN涂层进行一次高能Ti等离子体轰击处理, 偏压调节为–800 V (Ti等离子体能量为1.6 keV), 轰击时间2 min. 沉积时间分别为125, 150, 190, 210和270 min, 制备得到不同厚度的TiN涂层. 为方便讨论实验结果, 不同沉积时间制备的涂层样品分别命名为TiN-125, TiN-150, TiN-190, TiN-210和TiN-270. TiN-125, TiN-150, TiN-190, TiN-210和TiN-270涂层制备期间高能Ti等离子体轰击次数分别为2, 2, 3, 3, 4. 图 2 厚TiN涂层的制备工艺示意图 Figure2. Schematic diagram of preparation process of thick TiNcoating.
图3为不同沉积时间下的TiN涂层的厚度. 沉积时间为125, 150, 190, 210和270 min的TiN涂层的厚度分别为25, 30, 35, 45和50 μm. TiN-125涂层和TiN-150涂层的沉积速率均为0.2 μm/min. TiN-190涂层、TiN-210和TiN-270涂层的沉积速率分别为0.194, 0.187和0.185 μm/min. TiN涂层的沉积速率随涂层的厚度增加呈减小趋势. 这是因为随着沉积时间的增加, 沉积在基片上的TiN涂层越来越厚, 基片的绝缘性增强, 基片表面正电荷聚集导致到达样品表面的离子能量和数量减少, 使沉积速率降低. 此外, 值得注意的是厚度为TiN-270的涂层的沉积速率最小, 但与TiN-125的涂层的最高沉积速率相比, 仅降低了7.5%, 表明在TiN沉积过程中, 周期性地进行高能离子轰击处理, 对涂层的沉积速率影响较小. 图 3 TiN涂层厚度随沉积时间的变化 Figure3. The evolution of thickness of TiN coatings with deposition time.
23.2.形貌结构分析 -->
3.2.形貌结构分析
采用X Pert PRO MPD衍射仪对涂层做XRD扫描分析. 图4为所有TiN涂层的XRD图谱及不同沉积时间下样品的晶粒尺寸. 晶粒尺寸根据谢勒公式计算得到, 半峰宽通过对(111)峰进行高斯拟合后获得. 所有的涂层表现出典型的面心立方结构, 涂层由TiN相和非化学计量TiN0.9相组成, 涂层的应变能较低导致TiN相沿(111)密排面择优取向生长[30]. 非化学计量比TiNx (x < 1)与TiN相比, 应变能小、韧性高且抗冲蚀性能优良[31,32], TiN0.9相的存在使制备的TiN硬质涂层具有一定的韧性. 随着沉积时间的增加, (111)衍射峰的强度逐渐升高, 半峰宽数值呈递减变化, 涂层的晶粒从16.57 nm逐渐增大到27.66 nm, 这与Hu等[33]报道的一致, 涂层厚度增加, 晶粒粗化效应明显, 涂层越厚, 表面晶粒尺寸越大. 此外, 可以观察到样品的TiN(111)峰位向高衍射角偏移, 这与涂层的残余应力随涂层增厚而逐渐减小有关. 图 4 (a) 不同沉积时间的TiN涂层的XRD谱图; (b) 不同沉积时间的TiN涂层的晶粒尺寸; (c) TiN-125涂层的XRD谱图 Figure4. (a) XRD patterns of all of the TiN coatings with different deposition time; (b) the grain size of all of the TiN coatings with different deposition time; (c) XRD patterns of TiN-125 coating.
由XRD结果可知, 在制备TiN-125, TiN-150, TiN-190, TiN-210和TiN-270涂层过程中, 高能离子轰击对涂层的物相组成及结构影响较小, 各涂层的物相组成基本相同, 选择TiN-125涂层进行XPS分析. TiN-125涂层的XPS图谱如图5所示, N 1s谱峰的结合能在395—400 eV之间, 经泰勒解谱后分别在396.9和398.67 eV附近出现两个分峰, 位于396.9 eV处的N峰对应TiN中的N原子[34], 398.67 eV处的N峰对应TiN0.9中的N原子[35], Ti和N等离子体形成TiN的同时, 也生成了韧性较好的TiN0.9, 这与XRD分析的结果一致. 磁过滤阴极弧制备的厚TiN涂层由于存在非化学计量TiN0.9, 使TiN涂层具有高硬度的同时提高了涂层韧性. 图 5 TiN-125涂层N 1s的XPS谱图 Figure5. XPS spectta of N 1s of TiN-125 coating.
用原子力显微镜观察涂层的表面形貌. 不同沉积时间的TiN涂层的表面形貌及粗糙度如图6所示, 测试的区域为1 μm × 1 μm. TiN-210和TiN-270涂层的粗糙度相同, 为10.5 nm. TiN-125, TiN-150和TiN-190涂层粗糙度均小于10 nm, 分别为6.37, 6.68和9.64 nm. 涂层的沉积时间越长, 粗糙度越大. 沉积时间超过150 min后, 涂层的沉积速率略微降低, 涂层表面的颗粒团聚现象增多, 大颗粒越来越多. 随着TiN涂层厚度增加, 涂层表面的晶粒粗化效应明显, 大尺寸晶粒增多, 且更易成为涂层生长核心, 沿柱状晶方向持续长大形成大颗粒, 增大涂层表面粗糙度. 对于厚度超过35 μm的厚涂层, 高能离子轰击打碎大晶粒的作用减弱, 造成TiN-210和TiN-270涂层的粗糙度趋于相同. 图 6 不同沉积时间下的TiN涂层的AFM图及表面粗糙度 (a) 125 min; (b) 150 min; (c) 190 min; (d) 210 min; (e) 270 min; (f) 不同TiN涂层的表面粗糙度 Figure6. The AFM and roughness of all of the TiN coatings with different deposition time: (a) 125 min; (b) 150 min; (c) 190 min; (d) 210 min; (e) 270 min; (f) roughness of all of the TiN coatings.