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氟锆酸钾添加浓度对铝基PEO涂层热物理性能的影响∗

  • 朱剑威1,2

    机 构：

    1. 中国科学院力学研究所 北京 100190

    2. 中国科学院大学材料科学与光电工程中心 北京 100049

    ×
  • ， 李国栋1,2

    机 构：

    1. 中国科学院力学研究所 北京 100190

    2. 中国科学院大学材料科学与光电工程中心 北京 100049

    ×
  • ， 马飞3

    机 构：

    3. 潍柴动力股份有限公司 潍坊 261061

    ×
  • ， 黄国龙3

    机 构：

    3. 潍柴动力股份有限公司 潍坊 261061

    ×
  • ， 齐少豹3

    机 构：

    3. 潍柴动力股份有限公司 潍坊 261061

    ×
  • ， 郭灵燕3

    机 构：

    3. 潍柴动力股份有限公司 潍坊 261061

    ×
  • ， 李光1,2

    机 构：

    1. 中国科学院力学研究所 北京 100190

    2. 中国科学院大学材料科学与光电工程中心 北京 100049

    ×
  • ， 夏原1,2

    机 构：

    1. 中国科学院力学研究所 北京 100190

    2. 中国科学院大学材料科学与光电工程中心 北京 100049

    ×

1. 中国科学院力学研究所 北京 100190；
2. 中国科学院大学材料科学与光电工程中心 北京 100049；
3. 潍柴动力股份有限公司 潍坊 261061；

Effects of K2FZr6 Concentration on Thermo-physical Properties of Aluminum Based PEO Coating

  • ZHU Jianwei1,2

    Affiliation：

    1. Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190 , China

    2. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049 , China

    ×
  • ， LI Guodong1,2

    Affiliation：

    1. Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190 , China

    2. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049 , China

    ×
  • ， MA Fei3

    Affiliation：

    3. Weichai Power Co., Ltd, Weifang 261061 , China

    ×
  • ， HUANG Guolong3

    Affiliation：

    3. Weichai Power Co., Ltd, Weifang 261061 , China

    ×
  • ， QI Shaobao3

    Affiliation：

    3. Weichai Power Co., Ltd, Weifang 261061 , China

    ×
  • ， GUO Lingyan3

    Affiliation：

    3. Weichai Power Co., Ltd, Weifang 261061 , China

    ×
  • ， LI Guang1,2

    Affiliation：

    1. Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190 , China

    2. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049 , China

    ×
  • ， XIA Yuan1,2

    Affiliation：

    1. Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190 , China

    2. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049 , China

    ×

1. Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190 , China；
2. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049 , China；
3. Weichai Power Co., Ltd, Weifang 261061 , China；

作者简介:

李光,男,1972年出生,博士,副研究员。主要研究方向为材料表面科学及薄膜技术。E-mail:lghit@imech.ac.cn;

夏原(通信作者),男,1963年出生,博士,研究员。主要研究方向为材料表面科学及薄膜技术。E-mail:xia@imech.ac.cn

中图分类号:TG148

文献标识码:A

DOI:10.11933/j.issn.1007-9289.20210208001

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目录contents

    摘要

    为了研究氟锆酸钾(K2ZrF6 )添加对铝基 PEO 涂层热物理性能的影响。 在硅酸盐-氢氧化钠电解液体系下,通过掺杂不同浓度的氟锆酸钾 (0, 0. 5, 1. 5 和 3 g / L),使用等离子体电解氧化技术(PEO)在纯铝基体表面制备得到了氧化铝-氧化锆复合陶瓷涂层。 采用 DSC 以及 LFA 对涂层的热物理性能(比热容、热导率)进行了表征,并通过 SEM、XRD、EDS 等测试手段分析不同添加浓度下铝基 PEO 陶瓷涂层的显微形貌以及相结构组成,对其变化趋势及对热物理性能的影响机理进行讨论与分析。 结果表明:氟锆酸钾掺杂后,涂层相结构主要由 α-Al 2O3 、γ-Al 2O3 、莫来石与 t-ZrO2 组成;随着添加浓度的增加,陶瓷涂层的生长速率不断提高、内部的微孔数量增多、氧化锆含量持续上升,涂层密度先下降后增加,涂层表面出现局部放电现象;锆元素掺杂后涂层比热容呈现出波动下降趋势,热导率数值显著降低,当氟锆酸钾添加浓度为 1 g / L 时,涂层的热导率最低,为 0. 148 W/ (m·K),较添加前降低了 72. 5%。 对于纯铝基体,在硅酸盐体系电解液引入氟锆酸钾成分可以有效降低涂层的热导率,提高涂层的隔热性能。

    Abstract

    In order to explore the influence of K2ZrF6 on thermal-physical properties and improve the thermal insulation performance of aluminum based PEO coating, in this paper, plasma electrolytic oxidation (PEO) was used to deposit Al 2O3 -ZrO2 ceramic coating on aluminum substrate with different K2ZrF6 addition ( 0, 0. 5, 1. 5and 3g / L) in the NaSiO3 -NaOH electrolyte. Thermo-physical properties of ceramic coating were tested by differential scanning calorimeter ( DSC) and laser flash met hod ( LFA), and the mechanism of that properties were discussed by analyzing the variation of surface / cross-section morphology, phase structure and element content using SEM, XRD andEDS. The result shows that ceramic coatings phase structure in K2ZrF6 containing electrolytes mainly consists of α-Al 2O3 , γ-Al 2O3 , mullite and t-ZrO2 ; with the increase of the K2ZrF6 , the growth rate and micro pore amounts increase, which means the micro-discharge is more intense through the reaction process; the density of coating decrease first and then increase, and the partial discharge appeared when the K2ZrF6 was 3 g / L. With the addition of K2ZrF6 , the specific heat capacity of the coating showed a fluctuating decreasing trend, and the heat conductivity decreased significantly. When the added concentration of K2ZrF6 was 1g / L, ceramic coating has the minimum heat conductivity of 0. 148 W/ (m·K), which is less than normal 8YSZ coating (1. 68 ~ 2 W/ ( m·K)) and decreased by 72. 5% compared with that before K2ZrF6 adding. Besides, the coating at 1g / L K2ZrF6 addition also has a lower volume heat capacity (681 kJ/ (m 3·K)) compared with 8YSZ ( >2500 kJ/ (m 3·K)) and SiRPA coating (1300 kJ/ (m 3·K)). As the thermal barrier coating for internal combustion engine, characteristic of low volume heat capacity can prevent intake air heating and reduce the heat loss. In conclusion, for aluminum substrate, the addition of K2ZrF6 in silicate electrolyte can increase the amount of micro-pores, effectively reduce the thermal conductivity and improve the thermal insulation performance of PEO coating.

  • 0 前言

  • 随着发动机技术的发展以及“国六” 法规全国范围内的普及,柴油发动机面临着更严格的排放标准[1-2],而降低排放的同时意味着燃烧室内爆发压力的增加[3],这对柴油发动机零部件的强度、可靠性以及热疲劳性能都提出了更高的要求。然而,传统铝合金材料已经远远无法适应高性能柴油机尤其是重型柴油机的工作环境[4]。其中,钢活塞因其具有机械强度高、热膨胀系数低、高温性能稳定等优势[5-7],可承受大马力发动机中更高的温度和爆发压力[8],是目前国内外公司在高性能中重型柴油机活塞材料的研究重点[9-11]。同时,为了进一步提高柴油机的热效率以及保护基体,通常需要在钢活塞表面制备一层热障涂层[12]。

  • 由于静止空气热导率仅为0.026W/(m·K),涂层中孔隙率的引入将显著的提高涂层的隔热性能, 因此高孔隙率结构是热障涂层发展的重要方向。 BERTRAND等[13]采用纳米团聚粉末制备了具备纳米尺度微孔的热障涂层,其热导率最低可达到0.7W/(m·K);NOBORU等[14]通过等离子体喷涂方法在钢活塞表面制备了一种高孔隙(60%)的高效二氧化锆复合涂层,但是由于当孔隙率超过设计要求时会严重影响涂层的力学性能[15],导致恶劣工作环境中其会在热应力作用下失效破坏;另外,由于喷涂技术中涂层与基体以机械结合为主[16],其结合强度受到了较大的限制。因此,等离子体电解氧化技术作为一种原位生长涂层技术, 以其冶金结合的优势[17],成为了解决高孔隙率与高结合力之间矛盾的重要途经之一。

  • 等离子体电解氧化(Plasma electrolytic oxidation, PEO)是一种在电解液中通过高压放电在阀金属表面形成基体元素为主的陶瓷化技术,具备结合力高、生长速率快、隔热性能好等典型优势[18-19],被广泛的用于内燃机零部件防护领域;同时,PEO技术中的微弧放电与气体喷射特征有利于萌生复杂的多孔结构[20],从而形成大量封闭气室。但是,由于钢基体本身无法进行常规PEO处理,国内外常用技术是通过热浸镀[21-22] 或激光熔覆[23-24] 等方法在钢基体表面预制镀铝结合层,然后在其表面形成钢-铝-陶瓷复合涂层,PEO的实际作用对象是镀铝层。目前而言,纯铝形成的氧化铝陶瓷涂层结构较为单一,其隔热性能还具备较大的优化空间,复合PEO涂层技术成为提高陶瓷涂层防护指标的重要策略。其中, 二氧化锆具有高熔点、低热膨胀系数、化学性质稳定[25-26],等性质,是重要的耐高温材料以及增韧强化掺杂组分。但是,目前复合PEO陶瓷涂层中的氧化锆掺杂研究主要以提高耐腐蚀性能与耐磨性能为主,对于锆元素复合PEO涂层的热物理性能研究较为缺乏。

  • 鉴于此,文中以纯铝为试验基体,通过扫描电子显微镜( SEM)、X射线衍射仪(XRD)、差示扫描量热仪(DSC) 以及激光导热仪( LFA) 等方法,研究分析氟锆酸钾(K2ZrF6)添加浓度对铝基PEO涂层生长特性、显微形貌、相结构以及热物理性能的影响。

  • 1 试验材料及方法

  • 试验基材为铸铝,尺寸为 Φ38mm×6mm的薄圆片,试样侧边开3mm的圆孔并攻丝,用于连接电源。试验前依次使用50~1500号的砂纸对试样表面打磨至哑光,并使用金属清洗剂、去离子水和无水乙醇超声清洗。干燥后使用光洁导线连接铝片悬挂浸入电解液中,以试样为阳极,电解槽壁为阴极,电解槽壁内设有循环冷却水管道,保证试验期间电解液温度低于35℃。工作期间使用自动搅拌系统对电解液进行匀速搅拌以保证溶液的均匀性并辅助降温。试验溶液采用硅酸盐系电解液,主要成分包括硅酸钠和氢氧化钠。电参数与电解液参数如表1所示,并在此基础上添加系列浓度的氟锆酸钾。前期试验证明,当添加量超过3g/L后,会导致基材表面在30min时出现严重的烧蚀性大火花,导致局部涂层破坏,所以试验氟锆酸钾添加浓度分别为0、0.5、 1和3g/L共4组。

  • 表1 基本电参数和电解液参数

  • Table1 Experimental electrolyte parameters and electrical parameters

  • 试验中使用涡流测厚仪测量陶瓷涂层的平均厚度;采用SUPRA55型号扫描电子显微镜( SEM) 对PEO表面和截面形貌进行表征,并且使用能谱仪(EDS)进行元素成分检测,面扫描时间均为5min; 使用D/max-2500/PC型号X射线衍射仪(XRD)对涂层的相组成进行分析(扫描速度:2 °/min);热物理参数方面,使用差示扫描量热仪(DSC)耐驰404C测量涂层的吸放热曲线,通过与标准物蓝宝石对比计算得到涂层的比热容;使用激光闪烁法(LFA)测量涂层的热扩散系数,并通过计算得到涂层的热导率,测试设备为为德国耐驰LAF467Hyper flash。热物理性能测试温度均采用活塞顶面的平均温度(300℃)。

  • 2 结果与讨论

  • 2.1 涂层生长速率

  • 图1 所示为涂层的生长速率变化曲线。可以看出,随着氟锆酸钾浓度的增加,涂层生长速率呈现上升趋势,当添加浓度为3g/L时,涂层的生长速率最高,约为未添加组生长速率的1.8倍。这主要是由于氟锆酸钾在电解液中会分解产生F +与Zr 4+,离子浓度的提高会增加PEO电解液的电导率,从而提高了电流的利用效率,促进了等离子电解氧化的反应进程,所以PEO过程中放电火花就更加剧烈和密集,涂层的生长速率也因此提高。

  • 图1 K2ZrF6 浓度对涂层生长速率的影响

  • Fig.1 Growth rate of PEO coatings with different K2FZr6 addition

  • 2.2 显微形貌

  • 图2 为不同氟锆酸钾浓度下PEO涂层的表面微观形貌。由图2可知,涂层表面凹凸不平,存在大量的孔洞及烧结堆砌物,涂层表面都呈现明显的 “火山口”结构,这是由于反应过程中微区放电形成的瞬时高温高压(10 3~10 4 K,10 2~10 3 MPa)将熔融态氧化铝从放电通道喷射而出,熔融物质遇冷凝固后产生固态堆砌物。随着氟锆酸钾添加浓度的不断增加,电解液的电导率提高,火花放电剧烈程度越来越高,增加了局部放电效应,所以涂层表面粗糙度增加。

  • 图2 K2FZr6 添加涂层表面形貌

  • Fig.2 Surface morphology of PEO coatings with different K2FZr6 addition

  • 另外,由于等离子体微区反应在高温高压环境下产生气体喷射现象[20],所以在陶瓷涂层烧结结构表面布满大量的微纳米孔洞,甚至产生大孔包含小孔的嵌套式孔洞,形成“蜂窝”结构(如图3)。

  • 涂层的截面形貌变化如图4所示,PEO陶瓷涂层与铝基体结合紧密,颜色较深、靠近基体内侧为致密层,靠近外侧的为具有孔隙结构的疏松层。可以看出,氟锆酸钾添加会显著影响涂层的截面形貌:当添加浓度较小( 0.25g/L) 时,涂层截面微孔数量降低、致密性较高,这是由于氟锆酸钾在电解液中提供了F-,而微量F- 可以提高涂层的均匀性[ 27];但是随着离子浓度的逐渐提高,电解液电导率上升,原位生长过程中的火花放电、气体喷射现象更加剧烈和密集。所以,涂层中微孔数量逐渐增多,并且开始出现孔径较大的微孔。

  • 图3 涂层表面微纳米/嵌套式孔洞

  • Fig.3 Micro-nano/nested pores on surface of PEO coating

  • 图4 不同浓度K2FZr6 添加PEO涂层截面形貌变化

  • Fig.4 Cross section morphology of PEO coatings with different K2FZr6 addition

  • 不同添加浓度涂层密度如图5所示。可以看出,当氟锆酸钾添加浓度为0.5g/L时,涂层的孔隙率有所降低,同时,氧化锆的密度为氧化铝的2倍左右,所以,随着涂层微孔结构的减少以及高密度氧化锆相的增加,涂层密度也随之提高;随着氟锆酸钾添加浓度的进一步提高,涂层孔隙率增加,又会导致涂层密度出现下降趋势。但是,当添加浓度为3g/L时,涂层密度并没有随孔隙率增加而降低,这主要是由于高密度氧化锆相的进一步增加,氧化锆含量对密度的影响超过了孔隙率对密度的影响,因此,在孔隙率与成分结构的相互作用下,涂层密度呈波动变化趋势。

  • 图5 K2FZr6 浓度变化对PEO涂层密度与截面孔隙率的影响

  • Fig.5 Density and porosity of PEO coatings with different K2FZr6 addition

  • 2.3 元素组成及相结构

  • 图6 为PEO涂层XRD图谱。可以看出,当未进行锆盐掺杂时,所获得的涂层结构主要由 α-Al2O3、γ-Al2O3 与莫来石组成,其中Al相为基体相。当氟锆酸钾添加浓度为0.5g/L时,由于添加量较小,涂层结构取向尚未发生明显的变化;随着添加浓度的增加,涂层中莫来石、α-Al2O3 晶体结构取向逐渐降低甚至消失,并且在20°~30°左右出现明显的非晶“馒头峰”,XRD图谱中也开始出现四方相二氧化锆(t-ZrO2)结构。该变化说明在PEO过程中锆盐中的Zr元粒子在电场作用下向基体迁移[26],并在高温高压下在基体表面反应生成了高温相t-ZrO2,同时,高弹性模量的Al2O3 相包裹在ZrO2 相周围,阻止t-ZrO2 向m-ZrO2 的转变,从而起到稳定t-ZrO2 的作用,因此涂层中氧化锆主要以t型结构存在。

  • 图6 不同浓度K2FZr6 添加涂层XRD图谱

  • Fig.6 XRD patterns of PEO coatings at different K2FZr6 addition

  • 图7 中左图为不同添加浓度涂层内锆元素原子百分比含量变化曲线,右图为涂层截面的锆元素面扫描分布图,可以看出,图6a中呈均匀散点分布的锆元信号为背底误差,涂层本身并没有锆元素存在; 当氟锆酸钾添加浓度为0.5g/L时,由于涂层中锆元素含量较少,涂层截面中锆元素分布散点与背底对比度并不明显,仅能简单的辨认出截面轮廓;随着添加浓度的增加,涂层内锆元素含量逐渐上升,涂层截面面扫中锆元素的相对强度也随之增大,当氟锆酸钾添加浓度为3g/L时已经可以在面扫描图像通过锆元素散点分布清晰地辨别出涂层的截面形貌,此时涂层内锆元素原子含量比1g/L时增加了近1.5倍,这是浓度为3g/L时涂层密度上升的重要原因。

  • 图7 不同浓度K2FZr6 添加涂层锆元素含量变化曲线(左)不同浓度K2FZr6 添加涂层截面锆元素面扫图像(右)

  • Fig.7 Zirconium content of PEO coatings with different K2FZr6 addition(left) cross section map scanning(Zr) data with different K2FZr6 addition(right)

  • 2.4 比热容

  • 比热容与材料的成分、结构、晶格振动等因素密切相关,由于PEO涂层中存在大量非晶物质及复杂的中间化合物,因此文中仅针对XRD图谱中的主要因素进行分析。图8为不同浓度氟锆酸钾添加的涂层比热容变化曲线,随着添加浓度提高,比热容呈下降趋势,这是由于氧化锆相的比热容低于氧化铝相, 随着氟锆酸钾添加浓度的提高,涂层内部氧化锆相含量不断增加,所以比热容呈降低趋势。同时,比热容曲线还呈现波动变化的特征,这是由于氟锆酸钾的添加导致PEO发放电特性发生改变,从而引起涂层内部的非晶取向以及铝-硅、锆-硅中间化合物组分含量的变化。因此,在多因素作用下,涂层比热容随锆盐添加浓度的增加呈波动降低趋势。

  • 图8 氟锆酸钾添加浓度对PEO涂层比热容的影响(300℃)

  • Fig.8 Specific heat capacity of PEO coatings with different K2FZr6 addition(300℃)

  • 结合密度与比热容数据,可以计算得到涂层的体积热容。体积热容反映的是相同厚度下热障涂层对温度变化的的热响应速度。研究发现,低热容涂层所产生的“波动隔热”效应可以减少内燃机热损失[28-29]。当氟锆酸钾添加浓度为1g/L时,涂层的体积热容为681kJ/(m 3·K),低于常用8YSZ涂层(>2 500kJ/(m 3·K)) 与SiRPA涂层(1 300kJ/(m 3·K)) [28,30]。

  • 2.5 热导率

  • 陶瓷涂层热导率的计算公式为

  • γ=αρc

  • 式中,ρ 为密度,c 为比热容,γ 和 α 分别为热导率和热扩散系数。

  • 图9 所示为涂层热导率变化曲线。涂层的热导率随氟锆酸钾添加浓度增加呈现先下降后上升的趋势,相比于未经掺杂的陶瓷涂层,锆盐添加后涂层热导率均有降低。其中,当添加浓度为1g/L时,涂层的热导率最低,为0.148W/(K·m)。当添加浓度为3g/L时,由于涂层密度与比热容的增加,并且较大的孔径以及孔隙率的增加会造成大量通孔结构,导致对流传热增强[31],从而导致了热导率的回升。

  • 图9 氟锆酸钾添加浓度对PEO涂层热导率的影响(300℃)

  • Fig.9 Heat conductivity of PEO coatings with different K2FZr6 addition(300℃)

  • 3 结论

  • (1)少量氟锆酸钾的添加可以增加PEO陶瓷涂层的致密性,随着浓度的增加,涂层的生长速率不断上升,微孔数量增加,涂层整体趋于疏松状态;而过量氟锆酸钾的添加,反而会造成涂层表面产生烧蚀性火花,会导致涂层粗糙度增加。

  • (2)在硅酸盐体系电解液中,添加氟锆酸钾的复合PEO涂层主要由 α-Al2O3、γ-Al2O3、莫来石与t-ZrO2 组成,随着添加浓度的增加,涂层内部ZrO2 相含量上升,α-Al2O3、莫来石等相结构取向降低, 并且产生更多的非晶物质。

  • (3)氟锆酸钾的添加可以显著降低涂层热导率。当添加浓度为1g/L时,涂层的热导率最低,为0.148W/(K·m)。这主要是由于其综合了氧化锆相的低比热容以及氧化铝相的低密度特点;同时,等离子体电解氧化中的气体喷射易于形成大量微纳米气室,有利于构成封闭空气层、提高涂层的隔热性能。

  • (4)涂层比热容随氟锆酸钾添加浓度的增加呈波动下降趋势,当添加量为1g/L时,PEO陶瓷涂层具有较低的体积热容,为681kJ/(m 3·K),低于常用8YSZ以及SiRPA涂层。

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    • [29] KAWAGUCHI A,IGUMA H,YAMASHITA H,et al.Thermo-swing wall insulation technology:A novel heat loss reduction approach on engine combustion chamber[J].SAE international,2016.

    • [30] SONG X W,XIE M,ZHOU F.High-temperature thermal properties of yttria fully stabilized zirconia ceramics[J].Journal of Rare Earths,2011,29(2):155-159.

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  • 基本信息

    中图分类号: TG148
    文献标识码: A
    DOI: 10.11933/j.issn.1007-9289.20210208001

    基金信息

    国家自然科学基金资助项目(51871230,51701229)；
    Supported by National Natural Science Foundation of China (51871230&51701229).；

    引用信息

    稿件历史

    收稿日期: 2021-02-08

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