熱噴涂陶瓷涂層的耐磨應(yīng)用及涂層結(jié)構(gòu)調(diào)控方法

陳 林, 楊冠軍, 李成新, 王豫躍, 雒曉濤, 張山林, 李長久

西安交通大學(xué) 金屬材料強(qiáng)度國家重點(diǎn)實(shí)驗(yàn)室, 西安710049

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熱噴涂陶瓷涂層的耐磨應(yīng)用及涂層結(jié)構(gòu)調(diào)控方法

陳 林, 楊冠軍, 李成新, 王豫躍, 雒曉濤, 張山林, 李長久

西安交通大學(xué) 金屬材料強(qiáng)度國家重點(diǎn)實(shí)驗(yàn)室, 西安710049

摘 要：因涂層材料適用范圍廣、基材適應(yīng)性強(qiáng)、工藝靈活等特點(diǎn), 熱噴涂陶瓷涂層作為一類新型耐磨涂層已經(jīng)在很多領(lǐng)域獲得成功應(yīng)用。然而, 現(xiàn)代工業(yè)發(fā)展對耐苛刻條件下嚴(yán)酷磨損的高性能耐磨涂層提出了越來越高的需求, 如何通過材料-工藝的整體技術(shù)體系進(jìn)行涂層結(jié)構(gòu)的有效調(diào)控, 成為涂層技術(shù)領(lǐng)域的重要研究課題之一。本文在簡要介紹熱噴涂陶瓷涂層作為耐磨涂層應(yīng)用現(xiàn)狀的基礎(chǔ)上, 提取出對涂層耐磨性具有普遍意義的層內(nèi)扁平粒子間界面結(jié)合這一重要的涂層結(jié)構(gòu)本質(zhì)特征, 明確了涂層內(nèi)扁平粒子間界面強(qiáng)化的基本思路, 闡述了基于界面同質(zhì)強(qiáng)化和界面異質(zhì)強(qiáng)化的兩條思路進(jìn)行層間結(jié)合界面強(qiáng)化的研究進(jìn)展, 以期為面向更高耐磨性能的熱噴涂陶瓷涂層的材料選擇、結(jié)構(gòu)設(shè)計(jì)以及工藝優(yōu)化提供有益參考。

關(guān)鍵詞：熱噴涂；陶瓷涂層；金屬陶瓷涂層；耐磨損；結(jié)構(gòu)調(diào)控

磨損是自然界存在的普遍現(xiàn)象之一。磨損不僅導(dǎo)致材料損失從而引發(fā)機(jī)械零件失效, 還消耗巨大的能源[1]。統(tǒng)計(jì)顯示：因磨損消耗的能源占到全世界生產(chǎn)能源的1/3到1/2[2]；全球每年與磨損有關(guān)的損失約占GDP的2%～7%[3]；中國2006年的磨損損失約為9500億元人民幣, 約占GDP的4.5%[4]。在冶金、礦山、化工、建材及航空航天等各個(gè)工業(yè)領(lǐng)域, 磨損導(dǎo)致了約80% 零部件的失效,給國民經(jīng)濟(jì)造成了巨大損失。由此可見, 易磨損件壽命低已成為制約一些支柱產(chǎn)業(yè)升級(jí)發(fā)展的基礎(chǔ)問題之一。因此, 開發(fā)高耐磨表面技術(shù)對于延長零部件和設(shè)備連續(xù)可靠運(yùn)行壽命從而推進(jìn)支柱產(chǎn)業(yè)升級(jí)發(fā)展具有十分重大的現(xiàn)實(shí)意義[5]。

磨損, 是指摩擦副表面做相對運(yùn)動(dòng)時(shí)不可避免的因機(jī)械與化學(xué)作用所發(fā)生的材料變形和脫落現(xiàn)象[6]。磨損發(fā)生在兩物體相對運(yùn)動(dòng)的表面, 而且是在很薄的一層工作表面上。磨損的形式很多, 主要有磨粒磨損、粘著磨損、沖蝕磨損、微動(dòng)磨損、疲勞磨損和腐蝕磨損以及它們之間的相互結(jié)合或轉(zhuǎn)化, 其中磨粒磨損失效所占比重超過50%。材料耐磨損性能存在多種影響因素, 包括自身材質(zhì)、摩擦對副、載荷與速度、摩擦方式、表面粗糙度及其他表面特性、潤滑狀態(tài)、溫度和濕度等, 因此材料磨損是各種因素共同作用的一種異常復(fù)雜的綜合結(jié)果[7]。

目前, 降低材料磨損的主要方式除添加液相或固相潤滑劑之外, 主要是進(jìn)行表面改性或表面涂層處理。常用的表面涂層技術(shù)有熱噴涂、物理/化學(xué)氣相沉積 (PVD/CVD)、溶膠-凝膠 (Sol-Gel)、表面涂覆等方法。陶瓷涂層材料具有熔點(diǎn)高、硬度高、化學(xué)和熱穩(wěn)定性好、抗高溫氧化、耐腐蝕等優(yōu)異性能, 是一類優(yōu)異的耐磨損涂層材料[8-12]。根據(jù)北美市場統(tǒng)計(jì), 在各類陶瓷涂層中, 熱噴涂陶瓷涂層所占份額超過了50%。這是因?yàn)闊釃娡糠椒ň哂袑w材料、形狀和尺寸幾乎沒有要求、涂層材料適應(yīng)性極廣、生產(chǎn)效率高、工藝靈活性強(qiáng)等優(yōu)越性。

1　熱噴涂制備陶瓷耐磨涂層的原理及涂層結(jié)構(gòu)特征

1.1熱噴涂基本原理與工藝特點(diǎn)

熱噴涂技術(shù), 通常是用火焰、電弧、等離子射流等熱源 (或動(dòng)力源) 將粉末狀 (或絲狀、棒狀) 材料加熱至熔融或半熔融狀態(tài)并加速形成高速熔滴, 高速撞擊基體經(jīng)過扁平化、快速冷卻凝固沉積在基體表面形成涂層。圖1為熱噴涂技術(shù)原理示意圖[13]。

圖1　熱噴涂技術(shù)原理示意圖[13]Figure 1 Schematic diagram of thermal spray technology[13]

一般而言, 只要具有熔融狀態(tài) (物理熔點(diǎn))、能形成熔融態(tài)粒子或擬熔融態(tài)粒子的材料, 均可通過熱噴涂制備涂層。噴涂方法一般按熱源或動(dòng)力源性質(zhì)進(jìn)行分類, 比如電弧噴涂、普通火焰噴涂、爆炸噴涂、等離子噴涂、超音速火焰噴涂、低壓等離子噴涂[14]、激光噴涂等, 以及近年來發(fā)展的冷噴涂[14-20]、真空冷噴涂[21-28]、等離子噴涂-物理復(fù)相沉積 (PS-PVD)[29-34]等新工藝。采用火焰噴涂制備陶瓷涂層是一種相對較為經(jīng)濟(jì)的工藝。與火焰 (最高溫度一般為采用乙炔時(shí)的3200°C) 相比而言, 等離子體可以具有更高的溫度, 等離子射流中心溫度可以高達(dá)10000°C以上。因此, 等離子噴涂工藝可以熔化所有具有物理熔點(diǎn)的材料, 因而在噴涂制備高熔點(diǎn)材料 (特別是陶瓷材料) 涂層方面具有突出的優(yōu)越性。

1.2熱噴涂涂層結(jié)構(gòu)

以熔融或半熔融的粉末顆粒為沉積單元的噴涂工藝, 基本都具有以下相似的沉積成形過程：高速熔滴撞擊基體后首先鋪展扁平化, 隨后快速冷卻凝固形成扁平粒子, 逐層累加堆積形成涂層。因而,熱噴涂陶瓷涂層呈現(xiàn)典型的層狀結(jié)構(gòu) (如圖2所示)[35-39]。此外, 對于如Cr2O3、Al2O3等典型的脆性陶瓷材料, 在冷卻收縮過程中, 由于材料本征的脆性, 扁平粒子內(nèi)會(huì)形成大量垂直于扁平粒子平面的網(wǎng)狀裂紋。通過電鍍銅的方法可直觀顯化等離子噴涂Al2O3涂層的典型層狀結(jié)構(gòu)[35,36,40], 如圖3所示, 其中白色部分為鍍?nèi)胪繉拥你~, 代表涂層中存在的孔隙。從圖中可以看出, 等離子噴涂Al2O3涂層是由各扁平粒子經(jīng)過重疊、堆積形成的層狀結(jié)構(gòu), 層與層之間僅存在有限的區(qū)域結(jié)合, 其余部分則為大量的層間孔隙。統(tǒng)計(jì)結(jié)果顯示, 對于Al2O3涂層, 在通常沒有專門基體預(yù)熱的條件下, 等離子噴涂涂層的扁平粒子層間結(jié)合率最高約32%；同時(shí)涂層中還存在大量的垂直裂紋和氣孔, 這些氣孔、層間孔隙和垂直裂紋構(gòu)成了涂層的多孔結(jié)構(gòu), 并最終決定了涂層的力學(xué)性能和物理性能[35, 41]。

2　熱噴涂陶瓷涂層耐磨損性能及其應(yīng)用

熱噴涂陶瓷耐磨涂層根據(jù)材料種類可分為氧化物和非氧化物兩大類。

氧化物耐磨涂層材料中使用較為廣泛的是Al2O3和Cr2O3。為了改善單組分氧化物陶瓷涂層 (如純Al2O3、Cr2O3等) 固有的高脆性、多孔隙以及較低的結(jié)合性能等缺陷, 通常添加低熔點(diǎn)TiO2或SiO2粉末形成多元復(fù)合粉末, 以改善粉末的噴涂工藝性能, 獲得性能更加優(yōu)異的復(fù)合氧化物陶瓷涂層。

非氧化物主要包括碳化物、氮化物、硼化物等陶瓷材料, 這些陶瓷經(jīng)常具有比氧化物更高的硬度和更佳的耐磨損性能。然而, 由于高溫氣化和分解等問題, 難以直接通過熔融方式制備涂層。進(jìn)一步考慮到復(fù)合提高材料塑、韌性問題, 一般加入Co、Ni等金屬粘結(jié)相以形成陶瓷/金屬復(fù)合材料涂層。常用的碳化物陶瓷耐磨涂層有WC-Co、Cr2C3-NiCr等。在500°C以下, WC-Co耐磨涂層材料具有優(yōu)異的綜合性能[42-47]；在500°C～900°C, Cr2C3-NiCr具有優(yōu)異的抗氧化和耐磨損性能[48-51]。

圖2　熱噴涂YSZ涂層典型的 (a) 低倍和 (b) 高倍斷面組織形貌[39]Figure 2 Typical (a) low magnification and (b) high magnification cross sectional microstructure for thermally sprayed YSZ coatings[39]

圖3　典型的Al2O3涂層經(jīng)過電鍍銅顯化后的組織結(jié)構(gòu)[35, 36, 40]Figure 3 Typical cross-sectional microstructure of copper-plated Al2O3coating[35, 36, 40]

2.1氧化物陶瓷涂層

Cr2O3、Al2O3等氧化物陶瓷涂層硬度和強(qiáng)度很高, 且摩擦系數(shù)低, 廣泛應(yīng)用于水泵密封環(huán)、柱塞、耐磨環(huán)、軸承面等表面耐磨零部件。

對等離子噴涂Al2O3、Al2O3-13wt% TiO2和Cr2O3等涂層的組織結(jié)構(gòu)和干顆粒磨損性能的研究結(jié)果表明：Cr2O3涂層硬度較高, Al2O3-13wt% TiO2涂層具有優(yōu)異的各向同性；在文獻(xiàn)條件下, Al2O3涂層與100Cr6配副具有最優(yōu)的干顆粒磨損性能, 而Cr2O3涂層與燒結(jié)Al2O3配副則具有最優(yōu)的耐磨性能[52]。同時(shí), 對熱噴涂Al2O3和Cr2O3涂層滑動(dòng)磨損性能的研究發(fā)現(xiàn)[53-57]：(1) Al2O3和Cr2O3涂層的磨損與載荷和滑動(dòng)速度有關(guān), 存在最優(yōu)的臨界載荷和速度；(2) 在一定的載荷和速度下, 涂層的磨損速率變化先減小、后穩(wěn)定、最后快速增加并伴隨著磨損機(jī)制的轉(zhuǎn)變。有研究[58-61]表明, 在一定條件下, 等離子噴涂Cr2O3涂層比Al2O3及Al2O3-TiO2涂層具有更為優(yōu)異的耐磨性能, 磨損失效機(jī)制主要為脆性微觀切削的磨粒磨損及扁平化噴涂顆粒的界面脫落, 這說明涂層的耐磨性能與扁平化噴涂顆粒層間的結(jié)合強(qiáng)度密切相關(guān)。

Al2O3和Cr2O3陶瓷涂層雖然具有良好的耐磨損性能和較低的摩擦系數(shù), 但抗沖擊性較差, 加入TiO2、MoS2等陶瓷材料和固體潤滑劑有利于改善其綜合性能。Al2O3-TiO2復(fù)合陶瓷涂層具有硬度高、耐高溫性和耐磨性優(yōu)良等優(yōu)點(diǎn), 廣泛用于航空、航天、汽車和化纖等行業(yè), 有效延長了部件的使用壽命, 是目前應(yīng)用最廣的一種復(fù)合氧化物陶瓷涂層。

圖4　等離子噴涂Al2O3-40wt% TiO2的斷面形貌[67]Figure 4 Cross section of plasma sprayed Al2O3-40wt% TiO2coating[67]

研究發(fā)現(xiàn), TiO2涂層/不銹鋼配副的質(zhì)量損失隨載荷增大而增大卻隨滑動(dòng)速度增大而減小, 表現(xiàn)出良好的高溫軟化性能[62]。同時(shí), TiO2可以降低Al2O3的熔點(diǎn), 提高粉末的熔化程度, 降低涂層的孔隙率, 提高涂層與基體材料以及Al2O3顆粒間的粘接強(qiáng)度, 從而提高了陶瓷涂層的力學(xué)性能, 顯示出優(yōu)異的耐磨性能[63,64]。隨著TiO2含量的增加, Al2O3-TiO2復(fù)合陶瓷涂層的硬度值和表觀孔隙率以及摩擦系數(shù)依次降低, 涂層的致密度和結(jié)合強(qiáng)度依次提高；Al2O3-13wt% TiO2涂層在低速低載條件下具有顯著的減摩抗磨性能, Al2O3-20wt% TiO2和Al2O3-40wt% TiO2涂層 (斷面形貌如圖4所示) 在高壓力、高轉(zhuǎn)速工況下, 具有優(yōu)良的干摩擦特性[65-69]。此外, 載荷、潤滑和滑動(dòng)速度對Al2O3-TiO2復(fù)合陶瓷涂層失效行為也具有顯著影響 (如圖5所示), 在中速中載的條件下, 磨損失效機(jī)制主要為輕微斷裂和顆粒剝落；在高速高載條件下, 主要磨損失效機(jī)制為涂層的斷裂和剝落[70-72]。

2.2碳/氮/硼化物陶瓷涂層

除氧化物陶瓷之外, 還有碳化物、氮化物、硼化物等具有超高的硬度、優(yōu)異的高溫強(qiáng)度以及耐化學(xué)腐蝕和抗熱震性等各種綜合優(yōu)異性能的其他陶瓷材料, 比如典型的WC、ZrN、BC、SiC、TiB2等超硬陶瓷材料。然而, 由于這些材料在熔化之前優(yōu)先發(fā)生分解或氣化, 因而難以直接采用熱噴涂熔融或半熔的方式制備涂層。為此, 人們提出了反應(yīng)等離子噴涂制備陶瓷涂層的方法,比如以金屬鈦或鋁為原料粉末加入氮?dú)庖苑磻?yīng)沉積形成氮化物陶瓷涂層, 但在如何實(shí)現(xiàn)完全反應(yīng)以及如何控制涂層結(jié)構(gòu)方面還有待進(jìn)一步的深入研究[73-81]。此外, 在室溫條件下, 基于固態(tài)顆粒的高速碰撞沉積也可制備上述陶瓷材料的涂層。與陶瓷材料高速碰撞時(shí)發(fā)生脆性破碎和反彈的慣常認(rèn)知相反, 當(dāng)陶瓷顆粒尺寸下降到亞微米乃至更小時(shí), 陶瓷顆粒的高速碰撞也可實(shí)現(xiàn)陶瓷材料與沉積表面 (實(shí)際的基體表面或者已沉積的涂層表面) 的碰撞結(jié)合, 從而實(shí)現(xiàn)陶瓷涂層的有效沉積。納米TiO2、TiN等陶瓷顆粒的真空冷噴涂沉積結(jié)果表明[21-28, 82-84]：在近室溫條件下, 真空冷噴涂能夠制備出高硬度、良好結(jié)合的、均勻致密的具有優(yōu)異綜合性能的陶瓷涂層, 圖6顯示的是真空冷噴涂TiN涂層的斷面組織。

圖5　載荷和滑動(dòng)速度對Al2O3-13wt% TiO2涂層磨損率的影響[71]Figure 5 Influence of load and sliding speed on the wear rate of Al2O3-13wt% TiO2coating[71]

圖6　真空冷噴涂納米TiN涂層的 (a) 表面形貌和 (b) 斷面形貌[82, 84]Figure 6 Typical (a) cross-sectional microstructure and (b) fractograph of vacuum cold sprayed TiN coatings[82,84]

2.3 陶瓷/金屬復(fù)合結(jié)構(gòu)涂層

為了有效發(fā)揮碳化物、氮化物、硼化物等超硬陶瓷材料的優(yōu)越性能, 可將這些陶瓷材料與一定量的金屬復(fù)合起來構(gòu)成陶瓷/金屬復(fù)合結(jié)構(gòu)材料 (即通常所說的金屬陶瓷或硬質(zhì)合金), 借助于金屬粘結(jié)相的熔化來實(shí)現(xiàn)陶瓷顆粒的沉積。這不僅避免了陶瓷材料高溫氣化或分解問題, 還合理回避了陶瓷材料的脆性問題。

使用最為廣泛的是以碳化鎢、碳化鉻為硬質(zhì)相的硬質(zhì)合金, WC-Co是540°C以下硬度最高的耐磨涂層, 具有良好的抗沖擊性、韌性及結(jié)合性能, 在許多工程領(lǐng)域都有廣泛應(yīng)用[85, 86]。采用爆炸噴涂可制備較為致密的WC-Co涂層, 涂層磨損率隨載荷和速度的增大而增大, 當(dāng)載荷超過40N時(shí), 出現(xiàn)磨損突變現(xiàn)象, 裂紋擴(kuò)展引起的脆性斷裂是涂層失效的主要原因[87-89]。為進(jìn)一步提高WC-12Co涂層的減摩耐磨性, 可以在WC-12Co合金噴涂粉末中添加不同比例的MoS2粉末；MoS2均勻分布于復(fù)合涂層中。當(dāng)MoS2含量為2% 時(shí), 復(fù)合涂層的硬度、致密度變化不大, 但摩擦系數(shù)和磨損率大幅度下降 (分別為WC-12Co涂層的50% 和36%), 顯著提高了耐磨性能[90]。

在沖蝕磨損條件下[48-51, 91-99], 對超音速火焰噴涂 (HVOF) 的WC-Co和Cr2C3-NiCr涂層沖蝕性能進(jìn)行研究所得到的結(jié)果表明：HVOF金屬陶瓷涂層具有致密的結(jié)構(gòu) (如圖7所示)；粒子間的層間開裂脫落仍是涂層失效的主要原因。許多研究表明[91, 96, 100, 101]：隨HVOF硬質(zhì)合金中的碳化物顆粒尺寸的減小, 涂層的耐磨損性能增強(qiáng), 納米結(jié)構(gòu)硬質(zhì)合金涂層有望大幅度提高耐磨損性能。超音速火焰噴涂金屬陶瓷涂層雖然具有優(yōu)良的耐磨性能, 但在高溫高速噴涂過程中, 會(huì)不可避免地發(fā)生碳化物的分解和溶解[49, 102, 103], 當(dāng)采用納米結(jié)構(gòu)粉末時(shí), 顯著的WC分解與溶解使得采用HVOF難以制備納米結(jié)構(gòu)金屬陶瓷[100]。[48]

圖7　超音速火焰噴涂 (HVOF) Cr3C2-NiCr涂層的斷面組織形貌Figure 7 Cross-section microstructure of the HVOF sprayed Cr3C2-NiCr coating at (a) low magnification and (b) high magnification[48]

為了解決碳化物分解和溶解問題, 可以采用冷噴涂技術(shù)[18-20,100,104-113]制備金屬陶瓷復(fù)合涂層。冷噴涂是基于固態(tài)粉末顆粒高速碰撞實(shí)現(xiàn)沉積的涂層制備方法。固態(tài)成形過程完全避免了碳化物的分解溶解問題, 因此對于納米結(jié)構(gòu)陶瓷金屬復(fù)合涂層的制備尤其具有突出的優(yōu)越性。

冷噴涂納米WC-Co等粉末制備耐磨涂層的研究表明：噴涂前后粒子未發(fā)生相變和脫碳行為(如圖8所示)[20]。采用同時(shí)含有微米WC和納米WC顆粒的雙尺度結(jié)構(gòu)WC-Co粉末, 通過冷噴涂可以制備雙尺度結(jié)構(gòu)WC-Co涂層；相比于HVOF制備的微米結(jié)構(gòu)WC-Co涂層 (如圖9所示), 雙尺度結(jié)構(gòu)WC-Co涂層同時(shí)具有高韌性和高硬度 (硬度高達(dá)1683 MPa ± 176 Hv, 韌性高達(dá)18.9 MPa ± 4.0 MPa m1/2[108]) (如圖10所示), 即呈現(xiàn)了同時(shí)強(qiáng)韌化現(xiàn)象, 顯著提高了涂層的耐磨損性能。

圖8　冷噴涂納米WC-12Co涂層及噴涂粉末的XRD圖譜[20]Figure 8 XRD pattern of the cold-sprayed coating compared with that of the raw powder[20]

圖9　超音速火焰噴涂和冷噴涂涂層典型的磨損表面形貌和耐磨性能比較：(a) 超音速火焰噴涂 (HVOF)微米WC-Co涂層；(b) 冷噴涂雙尺度結(jié)構(gòu)WC-Co涂層；(c) 噴涂后退火處理的超音速火焰噴涂 (HVOF)微米WC-Co涂層；(d) 噴涂后退火處理的冷噴涂雙尺度結(jié)構(gòu)WC-Co涂層[104]Figure 9 Typical worn coating surface:(a) HVOF-sprayed micro WC-Co; (b) cold-sprayed WC-(nano WC-Co); (c) annealed HVOF micro WC-Co; (d) annealed WC-(nano WC-Co)[104]

圖10　(a) 不同WC涂層硬度與韌性關(guān)系比較以及 (b) 冷噴涂雙尺度結(jié)構(gòu)WC–Co涂層典型的裂紋橋接(左箭頭) 和曲折開裂路徑 (右箭頭) 形貌[108]Figure 10 (a) Relationship between hardness and toughness of WC–(nano WC–Co) in comparison with the reported WC–12Co and (b) typical crack morphology in WC–(nano WC–Co).The left inset in (b) shows the crack bridging by micro WC strengthening particles, and the right inset shows the bimodal-sized zigzag morphology of the fracture surface[108]

3　熱噴涂陶瓷涂層耐磨損性能的影響因素分析

在磨損服役條件下, 熱噴涂陶瓷涂層磨損的工況存在載荷、速度、配副、潤滑劑、溫度等多種組合, 而涂層的屬性包括自身的韌性、硬度、結(jié)合強(qiáng)度、散熱性能、微觀結(jié)構(gòu) (如孔隙、非晶相、表面織構(gòu))、熱穩(wěn)定性等諸多因素。因此, 涂層的磨損行為十分復(fù)雜, 是磨損機(jī)制、工況和自身屬性三部分交互作用的結(jié)果。通過對涂層摩擦磨損機(jī)理的研究, 熱噴涂陶瓷涂層在磨損應(yīng)用中損傷及失效的主要原因可以歸納為以下幾方面[114]:

(1) 噴涂材料及涂層的硬度不高, 在服役過程中快速磨損并最終失效；

(2) 涂層中夾雜的未熔化噴涂顆粒使得涂層內(nèi)局部結(jié)合不良并且增加了涂層的孔隙率,使得涂層容易產(chǎn)生裂紋, 進(jìn)而導(dǎo)致涂層失效；

(3) 涂層的殘余應(yīng)力較大, 使涂層容易產(chǎn)生裂紋及剝層脫落而失效；(4) 涂層與基體的結(jié)合強(qiáng)度不高, 使得涂層剝落而失效；

(5) 陶瓷噴涂材料韌性差, 磨損容易產(chǎn)生脆裂, 從而導(dǎo)致涂層失效。

研究發(fā)現(xiàn)[115, 116], 熱噴涂涂層在快速凝固冷卻過程產(chǎn)生了極大的 (GPa量級(jí)) 淬火應(yīng)力, 導(dǎo)致產(chǎn)生大量的層內(nèi)開裂以及分布極不均勻的殘余應(yīng)力。極大的淬火應(yīng)力的存在同時(shí)也使得涂層內(nèi)扁平粒子層間結(jié)合有限 (傳統(tǒng)常溫噴涂涂層的層間結(jié)合率最大僅為32%), 易發(fā)生層間脫落失效[36, 37]。

對涂層耐沖蝕性能與層間平均結(jié)合率之間關(guān)系的定量研究發(fā)現(xiàn)[40, 61, 117, 118], 陶瓷涂層的顆粒沖蝕性能主要由粒子間結(jié)合所控制, 而陶瓷涂層的ACT-JP值 (磨損速率的倒數(shù)) 正比于粒子間結(jié)合率(如圖11所示), 因此提高層間結(jié)合率和結(jié)合強(qiáng)度、降低殘余應(yīng)力可以有效地提高涂層耐磨性能。

因此, 改善層間結(jié)合是增強(qiáng)涂層耐磨性能的有效手段。提高涂層內(nèi)部的扁平粒子間結(jié)合率, 對于熱噴涂陶瓷涂層而言, 不僅僅關(guān)乎涂層的耐磨損性能, 同時(shí)對于涂層的導(dǎo)電等其他物理性能和服役功能也具有重要價(jià)值。近年來, 大量的研究工作都集中于改善熱噴涂陶瓷涂層的層間結(jié)合性能。下文中將指出, 這些工作已經(jīng)取得了顯著進(jìn)展。

4　熱噴涂陶瓷涂層的層間結(jié)合調(diào)控方法

4.1界面同質(zhì)強(qiáng)化——通過噴涂工藝優(yōu)化實(shí)現(xiàn)界面結(jié)合直接調(diào)控

為了改善層間結(jié)合, 一些高能束重熔方法被用于強(qiáng)化層間結(jié)合乃至實(shí)現(xiàn)完全結(jié)合, 激光重熔處理[119-125]就是其中之一。然而, 激光重熔處理將引發(fā)基體材料的高溫?fù)p傷或變形, 因此只適用于對基體材料沒有嚴(yán)格要求的少量場合。

圖11　涂層的扁平粒子間平均結(jié)合率與沖蝕抗力之間的關(guān)系[40]Figure11 Effect of the mean bonding ratio of the inter-splats on the erosion resistance[40]

為了提高涂層的結(jié)合率和性能, 很多學(xué)者致力于通過噴涂工藝參數(shù)的調(diào)控來改變噴涂粒子的狀態(tài) (尤其是噴涂粒子的速度和溫度), 以期提高涂層的層間結(jié)合率。有學(xué)者[36, 38, 126-131]研究了等離子噴涂距離對涂層結(jié)合率的影響, 結(jié)果表明：隨著噴涂距離的改變, 涂層結(jié)合率增加很小 (如圖12所示),最大結(jié)合率僅為32%, 這說明改變噴涂距離無法有效提高涂層的結(jié)合率。關(guān)于噴涂方法對涂層結(jié)合率影響的研究[38, 132-134]發(fā)現(xiàn)：噴涂方法對粒子的速度影響顯著, 但提高粒子速度并未能增加結(jié)合率(參見表1)。此外, 許多學(xué)者[126, 131, 134]研究了等離子噴涂功率對涂層層間結(jié)合率的影響, 結(jié)果表明：只要噴涂粒子熔化良好, 粒子層間結(jié)合率不隨功率的提高而顯著增加 (如圖13所示), 這說明提高功率也不能有效地提高涂層的層間結(jié)合率。通過對等離子噴涂粒子的溫度和速度之間的關(guān)系的研究也發(fā)現(xiàn)[126, 128, 135-137]：增加粒子溫度有助于增加界面結(jié)合, 然而增加粒子速度卻會(huì)導(dǎo)致粒子在束流中的駐留時(shí)間縮短進(jìn)而降低粒子溫度, 增加功率會(huì)增加粒子速度而不顯著改變粒子溫度；因此, 粉末在加速過程中溫度和速度難以同時(shí)提高, 改變噴涂距離和功率難以實(shí)現(xiàn)粒子溫度的有效增加, 這使得傳統(tǒng)的改變噴涂參數(shù)進(jìn)而提高結(jié)合率的思路迄今尚難以實(shí)現(xiàn)。

圖12　噴涂距離對(a) Al2O3涂層[126]和 (b) YSZ涂層[128]平均結(jié)合率的影響Figure 12 Effect of spray distance on the mean bonding ratio of plasma-sprayed (a) Al2O3coatings[126]and (b) YSZ coatings[128]

表1　不同噴涂方法的粒子速度和平均結(jié)合率比較Table 1 Comparison of the particle velocity and mean bonding ratio of different spraying methods

圖13　噴涂功率對等離子噴涂Al2O3涂層平均結(jié)合率的影響[126]Figure 13 Effect of plasma arc power on the mean bonding ratio of plasma-sprayed Al2O3coatings at a spray distance of 100 mm[126]

圖14　不同基體溫度下的YSZ涂層的 (a) 結(jié)合率以及 (b～d) 斷面形貌。(b)、(c)、(d) 分別對應(yīng)于基體溫度為室溫、686oC、1100oC)[39, 145]Figure 14 (a) The dependence of mean bonding ratio for YSZ coatings on deposition temperature and different cross sectional microstructures deposited at (b) room temperature, (c) 686oC and (d) 1100oC, respectively[39, 145]

圖15　YSZ涂層的 (a) 彈性模量、(b) 硬度、(c) 斷裂韌性、(d) 電導(dǎo)率與沉積溫度的關(guān)系[39]Figure 15 The relationship between (a) elastic modulus, (b) hardness, (c) fracture toughness, (d) electrical conductivity and deposition temperature for YSZ coatings[39]

考慮到界面結(jié)合實(shí)際上是單個(gè)液滴碰撞沉積整個(gè)過程形成的結(jié)果, 除液滴狀態(tài)參數(shù)之外,沉積基體 (主要指前序已沉積的涂層表面) 表面也是參與形成層間結(jié)合的配副。研究[39, 138-144]發(fā)現(xiàn), 改變基體溫度確實(shí)可以有效地改變粒子的結(jié)合狀態(tài), 并存在一個(gè)臨界結(jié)合溫度使得大量噴涂陶瓷涂層扁平粒子形成有效的界面結(jié)合 (Al2O3和YSZ陶瓷涂層的臨界結(jié)合溫度分別為300°C 和600°C)；當(dāng)基體溫度超過臨界結(jié)合溫度, 涂層結(jié)合率隨沉積溫度提高而顯著增加, 扁平粒子間甚至形成跨粒子界面的連續(xù)生長 (如圖14所示)。隨著基體溫度的升高, 扁平粒子間實(shí)現(xiàn)了跨界面連續(xù)生長, 涂層的彈性模量、顯微硬度、斷裂韌性和電導(dǎo)率都表現(xiàn)出顯著的增加 (如圖15所示), 顯示出更加優(yōu)異的力學(xué)性能[39, 138-141]。

4.2界面異質(zhì)強(qiáng)化——借助于后處理實(shí)現(xiàn)界面結(jié)合調(diào)控

在磨損過程中, 等離子噴涂層固有的孔隙往往作為疲勞裂紋萌生的核心, 最終導(dǎo)致整個(gè)涂層磨損失效。然而, 這些相互貫通的孔隙, 也恰好是可以借助于第二相材料浸滲到涂層內(nèi)部從而實(shí)現(xiàn)界面異質(zhì)強(qiáng)化的通道。根據(jù)浸滲處理溫度的不同, 主要有高溫浸滲處理[146-153]和低溫浸滲處理[154-156]。高溫浸滲工藝需要將涂層和滲透劑加熱至很高的溫度 (近熔點(diǎn)), 這極大地限制了高溫處理在工業(yè)中的應(yīng)用。通過向等離子噴涂Al2O3涂層內(nèi)部低溫滲膠, 有效擴(kuò)展了滲透處理在等離子噴涂耐磨層中的應(yīng)用[154, 155]。研究表明[154, 155], 低溫滲膠可以極大地減少陶瓷涂層內(nèi)部的裂紋孔隙 (如圖16所示), 有效地減少了失效裂紋源, 同時(shí)顯著地增加了涂層的層間結(jié)合率和層內(nèi)結(jié)合強(qiáng)度, 使涂層的結(jié)合強(qiáng)度從約不足10 MPa水平提高到超過50 MPa的水平, 耐沖蝕性能可提高約10倍。

圖16　等離子噴涂氧化鋁涂層滲膠前后涂層的斷面形貌及耐沖蝕性能[154]：(a) 滲膠前的斷面組織；(b) 滲膠后的斷面組織；(c) 滲膠前后的沖蝕曲線；(d) 滲膠前后在不同沖蝕角度條件下的沖蝕磨損率比較Fgure 16 Microstructure of (a) as-sprayed and (b) infiltrated Al2O3coating, and comparison of (c) the total weight loss and (d) the erosion rate of as-sprayed Al2O3coating with those of the adhesive-infiltrated coatings (Type-A and Type-B means two different adhesives)[154]

5　結(jié)束語

本文在簡要介紹了熱噴涂陶瓷涂層耐磨損應(yīng)用的基礎(chǔ)上, 提取出對涂層耐磨性能具有最重要影響和普遍意義的層內(nèi)扁平粒子間界面結(jié)合 (層間結(jié)合率和結(jié)合強(qiáng)度) 這一重要的涂層結(jié)構(gòu)本質(zhì)特征,明確了涂層內(nèi)扁平粒子間界面強(qiáng)化的基本思路, 闡述了基于提高基體溫度的界面同質(zhì)強(qiáng)化和低溫滲透處理的界面異質(zhì)強(qiáng)化的研究進(jìn)展, 以期為面向高性能耐磨涂層的熱噴涂陶瓷涂層的材料選擇、結(jié)構(gòu)設(shè)計(jì)以及工藝優(yōu)化提供有益的參考。研究表明：提高沉積溫度可以實(shí)現(xiàn)跨扁平粒子界面的連續(xù)生長、顯著提高層間結(jié)合率；對等離子噴涂陶瓷涂層進(jìn)行低溫浸滲處理可以顯著減少作為失效裂紋源的扁平粒子層內(nèi)層間孔隙, 同時(shí)有效提高涂層的層間層內(nèi)結(jié)合強(qiáng)度, 進(jìn)而大幅提高陶瓷涂層的耐磨性能；同時(shí), 采用冷噴涂微/納米金屬陶瓷粉末制備微/納雙尺度結(jié)構(gòu)金屬陶瓷涂層具有均勻致密的組織結(jié)構(gòu)和極高的韌性/硬度 (硬度與致密塊材相當(dāng)), 即涂層呈現(xiàn)奇異的同時(shí)強(qiáng)韌化現(xiàn)象, 顯著提高了涂層的耐磨損性能, 因而具有廣闊的應(yīng)用前景。

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第一作者陳林, 男, 1990年出生。2009年9月進(jìn)入西安交通大學(xué)材料學(xué)院攻讀學(xué)士學(xué)位, 于2013年9畢業(yè)并被保送繼續(xù)攻讀碩士研究生, 2014年開始碩博連讀, 導(dǎo)師楊冠軍教授。博士階段主要從事于等離子噴涂陶瓷涂層的微觀結(jié)構(gòu)形成機(jī)理、涂層結(jié)構(gòu)與力學(xué)性能關(guān)系理論方面的基礎(chǔ)研究, 以期為未來高性能熱障涂層的結(jié)構(gòu)設(shè)計(jì)、材料選擇和性能評(píng)價(jià)提供理論基礎(chǔ)。

Thermally Sprayed Ceramic Coatings for Wear-Resistant Application and Coating Structure Tailoring Towards Advanced Wear-Resistant Coatings

CHEN Lin, YANG Guan-Jun, LI Cheng-Xin, WANG Yu-Yue, LUO Xiao-Tao, ZHANG Shan-Lin, LI Chang-Jiu

State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China

Abstract:Thermal spraying has been widely applied to prepare wear-resistant ceramic coatings due to its flexibility to coating materials, substrate materials and processing.However, with the development of modern industry, the wear-resistant performance of coatings is required to be further improved to fulfill the increasingly harsh wear condition.How to effectively tailor coating structure towards a high wear resistance becomes a key issue.In this paper, the application of thermally sprayed wear-resistant ceramic coatings was briefly summarized, and a most important structural factor, inter-lamellar bonding state, influencing the wear resistance was proposed in this paper.The methods to strengthen the inter-lamellar bonding via homogeneous interface strengthening and heterogeneous interface strengthening were discussed towards a higher wear-resistant performance based on the material selection, structure design and processing parameter optimization.

Key words:Thermal spray; Ceramic coatings; Metal matrix ceramic composite coatings; Wearresistance, Structural tailoring

通訊作者：楊冠軍 (1977 -), 男, 河北唐山人, 教授、博士生導(dǎo)師。E-mail:ygj@mail.xjtu.edu.cn。 楊冠軍, 男, 教授/博導(dǎo), 1977年出生。2013年入選首批“國家萬人計(jì)劃”, 2008年入選教育部“新世紀(jì)優(yōu)秀人才計(jì)劃”。從事先進(jìn)涂層和特種陶瓷技術(shù)等研究開發(fā), 主持各類科研課題10余項(xiàng), 發(fā)表SCI論文90余篇, 獲授權(quán)發(fā)明專利20余項(xiàng), 獲省部級(jí)和國際學(xué)術(shù)獎(jiǎng)勵(lì)5項(xiàng), 應(yīng)邀做國際國內(nèi)學(xué)術(shù)會(huì)議邀請報(bào)告、任會(huì)議組委會(huì)委員或分會(huì)主席等20余次。兼任中國硅酸鹽學(xué)會(huì)特陶分會(huì)理事、中國機(jī)械工程學(xué)會(huì)表面工程分會(huì)委員、中國硅酸鹽學(xué)會(huì)測試技術(shù)分會(huì)理事、中國腐蝕與防護(hù)學(xué)會(huì)高溫專業(yè)委員會(huì)委員。

基金項(xiàng)目：國家萬人計(jì)劃專項(xiàng)基金。

收稿日期：2015-12-25 收到修改稿日期：2016-01-29

文獻(xiàn)標(biāo)識(shí)碼：A

DOI：10.16253/j.cnki.37-1226/tq.2016.01.002

中圖分類號(hào)：TB383

文獻(xiàn)編號(hào)：1005-1198 (2016) 01-0003-19

第一作者： 陳 林 (1990 -), 男, 陜西商洛人, 博士研究生。E-mail:chenlin.09021034@stu.xjtu.edu.cn。