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May 7, 2018 - Materials 2018, 11, 749; doi:10.3390/ma11050749 ..... magnesia partially stabilized zirconia refractory. Ceram. Int. 2016, 42, 10593–10598.
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High-Temperature Wettability and Interactions between Y-Containing Ni-Based Alloys and Various Oxide Ceramics Jinpeng Li 1 , Huarui Zhang 1,2, * and Hu Zhang 1,2, * 1

2

*

ID

, Ming Gao 1 , Qingling Li 1 , Weidong Bian 1 , Tongxiang Tao 2

School of Materials Science and Engineering, Beihang University, Beijing 100191, China; [email protected] (J.L.); [email protected] (M.G.); [email protected] (Q.L.); [email protected] (W.B.) Qingdao Institute of New Material technology of Beihang University, Qingdao 266000, China; [email protected] Correspondence: [email protected] (Hua.Z.); [email protected] (Hu.Z.); Tel.: +86-10-8233-9256 (Hua.Z.)

Received: 19 April 2018; Accepted: 4 May 2018; Published: 7 May 2018

 

Abstract: To obtain appropriate crucible materials for vacuum induction melting of MCrAlY alloys, four different oxide ceramics, including MgO, Y2 O3 , Al2 O3 , and ZrO2 , with various microstructures were designed and characterized. The high-temperature wettability and interactions between Ni-20Co-20Cr-10Al-1.5Y alloys and oxide ceramics were studied by sessile drop experiments under vacuum. The results showed that all the systems exhibited non-wetting behavior. The contact angles were stable during the melting process of alloys and the equilibrium contact angles were 140◦ (MgO), 148◦ (Y2 O3 ), 154◦ (Al2 O3 ), and 157◦ (ZrO2 ), respectively. The interfacial reaction between the ceramic substrates and alloys occurred at high temperature. Though the ceramics had different microstructures, similar continuous Y2 O3 reaction layer with thicknesses of about 25 µm at the alloy-ceramic interface in MgO, Al2 O3 , and ZrO2 systems formed. The average area percentage of oxides in the alloy matrices were 0.59% (MgO), 0.11% (Al2 O3 ), 0.09% (ZrO2 ), and 0.02% (Y2 O3 ), respectively. The alloys, after reacting with MgO ceramic, had the highest inclusion content, while those with the lowest content were in the Y2 O3 system. Y2 O3 ceramic was the most beneficial for vacuum induction melting of high-purity Y-containing Ni-based alloys. Keywords: high temperature ceramic; high-temperature wettability; interfacial interaction; MCrAlY alloys

1. Introduction With the development of the aviation industry, the inlet temperature of military large-scale engine turbines has been increased to 1850–2000 K. The working temperature of hot-end components of aero-engines is getting higher and higher, and is affected by the oxidation and corrosion of high-temperature gas [1,2]. MCrAlY (M = Ni and/or Co) alloys are widely used as overlays or bond coats for thermal barrier coatings (TBCs) to protect gas turbine blades and other hot components against high-temperature oxidation and hot corrosion [3,4]. At present, the common methods for preparing superalloys include vacuum induction melting (VIM), vacuum arc melting (VAR), electroslag remelting (ESR), electron beam remelting (EBR), and a combination of the two or three methods [5,6]. VIM has been almost the most common method for nickel-based superalloys. However, VIM is the only vacuum melting method that uses a refractory crucible made of oxides such as SiO2 , CaO, Al2 O3 , ZrO2 , and MgO [7,8]. Unfortunately, the refractory is a contamination source of oxygen because the metal/refractory interface has a thermal mechanical Materials 2018, 11, 749; doi:10.3390/ma11050749

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infiltration and thermophysical effects, which lead to crucible disintegration [9,10]. Moreover, during the melting process, interfacial chemical reactions may occur between the ceramics and the active elements, such as Ti, Hf, Al, Cr, and so on. Reaction products precipitate at the alloy-ceramic interface and within the alloy matrix, consequently deteriorating the microstructure and mechanical properties of the resultant Ni-based alloys [11–14]. Qing [15] found that Cr, Hf levels had the effect of accelerating interface reaction between IC6, K465, K4648, K488, and K002 alloy melts and silicon oxide core, and reaction products in alloys were mainly hafnium oxides and alumina. Kanetkar et al. [16] and Valenza et al. [17] observed the phenomenon that oxides were formed on the surface of a zirconia substrate after sessile drop experiments. However, the reason for the formation of oxides was not clear and the information of wetting behavior under high temperature between them was severely deficient. The wettability of ceramics with regard to molten alloys is of importance to understand the reaction mechanisms between the alloys and ceramics [18,19]. Previously, Jiang et al. [20] analyzed the mechanisms of the interfacial reactions between Zr-containing nickel-based superalloy and magnesia partially stabilized zirconia ceramic by means of wetting experiments. They found that the contact angle was larger than 90◦ during the continuous temperature rise, the equilibrium contact angle was 92◦ , and the interfacial reaction was influenced by the reduction of ZrO2 . A recent work by Chen et al. [21] showed that the interface reactions between Ni3 Al based superalloy and Al2 O3 ceramic occured as C and Hf contents reached a critical value. In addition, adsorptions of Hf and interface reactions improved the wettability obviously. Kritsalis et al. [22] also reported that Ni alloys containing up to 20 at % Cr, wetting and thermodynamic adhesion to alumina were mainly determined by the concentration of oxygen dissolved in the alloys. Thus, an appropriate refractory is of importance with regard to the wetting process, as well as the interfacial reactions between the alloys and ceramics. It is well established that an element, such as aluminum, in the alloy must be selectively oxidized to form a continuous external scale which is resistant to cracking and spalling. The adherence of the oxide scale is crucial to oxidation resistance. Some elements, such as yttrium, hafnium, and cerium (oxygen active elements), can be used to very substantially improve oxide scale adherence [23]. MCrAlY alloys contain Y at concentrations and Y improves the adherence of Al2 O3 scales to these coatings. To obtain high-quality MCrAlY alloys, ceramics with appropriate materials are essential. This will contribute to controlling appropriate levels of wettability and reducing interfacial reactions with the Ni-based alloys during melting. Unfortunately, the influence of various oxide ceramics on high-temperature wettability and interactions between Y-containing Ni-based alloys and ceramics have rarely been investigated. In this paper, four different oxide ceramics, including MgO, Y2O3, Al2O3, and ZrO2, are self-designed and characterized. The influence of various ceramics on the high-temperature wettability and interfacial reactions with regard to Ni-20Co-20Cr-10Al-1.5Y alloys and ceramics were studied. Meanwhile, the corresponding mechanisms were clarified. 2. Materials and Methods The Ni-20Co-20Cr-10Al-1.5Y (wt %) master alloys were prepared by a 100 KW ZG125C vacuum induction furnace (Herz Special Metallurgy Plant, Shanghai, China) equipped with appropriate amounts of high-purity metals, Ni (99.98%), Co (99.50%), Cr (99%), Al (99.99%), and Y (99.9%), as raw materials. The 3 mm × 3 mm × 3 mm cubes (about 0.16 g) obtained from the central part of the master alloys were used for the wetting experiments. The raw material particle size and some specific parameters were designed and given in Table 1. Polyvinyl alcohol (PVA) as a binder was used to prepare ceramic powder and milled for 2 h, and then dried at the controlled temperature of 298 K and the relative humidity of 30 ± 1% for 1 h. Four different oxide ceramics were prepared in dimensions of Φ 21 mm × 10 mm by the dry-pressing method using a uni-axial pressure machine (YLJ-40T, Shenyang Kejing Automation Equipment Co., Ltd., Shenyang, China) and sintered using a high-temperature box resistance furnace. The sintering temperature is 1913 K, the heating rate is 20 K/min, and furnace cooling is performed. After sintering

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temperature is 1913 K, the heating rate is 20 K/min, and furnace cooling is performed. After sintering and, consequently, before performing the contact angle experiments, the substrate surface was and, consequently, before performing the contact angle experiments, the substrate surface was ground ground and polished in order to control the roughness at the S/L interface. and polished in order to control the roughness at the S/L interface. Table 1. Parameters related to substrates with various oxide ceramics. Table 1. Parameters related to substrates with various oxide ceramics.

Parameters Parameters Purity (%) Purity (%) Particle size (mesh) Particle size (mesh) Grinding time (min) Grinding time (min) PVA binder (wt %) PVA binder (wt %) Molding pressure (MPa) Molding pressure (MPa) porosity Open Open porosity (%) (%)

MgO MgO 99.9 99.9 500 500 10 10 55 10 10 11.8 11.8

Y2O3

Al2O3

ZrO2 94 99.9325 99.9 200–30094 200 325 10 200–30010 200 10 10 10 10 5 5 5 5 5 5 10 10 10 10 10 10 20.44 20.44 24.4124.4126.42 26.42

Y2 O99.9 Al2 O399.9 ZrO2 3

The wetting wetting experiments experiments between between the the ceramics ceramics and and alloys alloys were were performed performed using using an an improved improved The sessile drop drop equipment, equipment, as as shown shown in in Figure Figure 1, 1, which which offered offered aa distinct distinct advantage advantage in in measuring measuring initial initial sessile contact angles and preventing the occurrence of the interfacial reactions between the alloys and oxide contact angles and preventing the occurrence of the interfacial reactions between the alloys and oxide ceramic substrates during heating. The whole experimental process was in a vacuum atmosphere to ceramic substrates during heating. The whole experimental process was in a vacuum atmosphere prevent active elements from this to prevent active elements fromevaporation evaporationand andoxidation, oxidation,and andthe theoxygen oxygenpartial partial pressure pressure in in this −8 Pa [24]. When the temperature was 1873 K, the alloy, which was condition should be lower than 10 − 8 condition should be lower than 10 Pa [24]. When the temperature was 1873 K, the alloy, which was located at was dropped. TheThe spreading process was was recorded by thebyCCD located at the the top topofofthe theequipment, equipment, was dropped. spreading process recorded the camera. The contact anglesangles were were directly measured fromfrom the captured dropdrop profiles using an CCD camera. The contact directly measured the captured profiles using axisymmetric-drop-shape analysis (ADSA) program with an error within ±1° [25]. ◦ an axisymmetric-drop-shape analysis (ADSA) program with an error within ±1 [25]. After the the sessile-drop sessile-drop experiments, experiments, the the solidified solidified samples samples were were embedded embedded in in resin resin and and then then cut cut After and polished for microstructural observations. The microstructure of the samples were analyzed and polished for microstructural observations. The microstructure of the samples were analyzed by by scanning electron electronmicroscope microscope (SEM, JSM6010, Electronics Co.,Tokyo, Ltd., Japan). Tokyo, The Japan). The scanning (SEM, JSM6010, JapanJapan Electronics Co., Ltd., chemical chemical compositions were by energy dispersive spectrometer (EDS, Oxford compositions were analyzed by analyzed energy dispersive spectrometer (EDS, Oxford INCAPentaFET-x3, INCAPentaFET-x3, Japan Electronics Co., Ltd., Tokyo, Japan). The phases of the samples were Japan Electronics Co., Ltd., Tokyo, Japan). The phases of the samples were identified by X-ray identified by X-ray microdiffraction (D8 Discover with GADDS, Bruker, Karlsruhe, Germany). microdiffraction (D8 Discover with GADDS, Bruker, Karlsruhe, Germany). The open porosity levelThe of openas-obtained porosity level of each as-obtained substrates wastomeasured (according to surface GB/T 1966each ceramic substrates wasceramic measured (according GB/T 1966-1996). The line 1996). The surface (Ra) of each ceramic substrate measured using a LEXT OLS4000 roughness (Ra) of line eachroughness ceramic substrate was measured usingwas a LEXT OLS4000 3D laser (12.5mm, 3D laser (12.5mm, performed three times, Olympus Tokyo, Japan). performed three times, Olympus Corporation, Tokyo,Corporation, Japan).

Figure Schematic diagram the improved Figure 1. 1. Schematic diagram of of the improved sessile sessile drop drop equipment. equipment.

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3. Results 3. Results 3.1. Microstructural of Ceramic Substrates 3.1. Microstructural of Ceramic Substrates The microstructures of polished surface including MgO, Y2O3, Al2O3, and ZrO2 substrates prior The microstructures of polished surface including MgO,was Y2Oa3,distinct Al2O3, and ZrO2 substrates priorthe to to the wetting experiments are shown in Figure 2. There difference in pores and the wetting of experiments are shownIninthe Figure There difference in pores and thecompletely exhibition exhibition ceramic particles. case2.of the was MgOa distinct substrate, MgO particles were of ceramictogether, particles.and In the case of thepores MgOcould substrate, MgO particles completely sintered sintered a few closed be observed withinwere the finely-sintered MgOtogether, blocks. and a few be observed the finely-sintered MgO In the case ofrelatively the Y2O3 In the caseclosed of thepores Y2O3could substrate, thoughwithin the degree of sintering of theblocks. Y2O3 particles was substrate, though of theYdegree of sintering of the Ytogether. was relatively the majority Y2O3 low, the majority 2O3 particles were sintered In the case of the Allow, 2O3 substrate, theofdegree 2 O3 particles particles were Inwas the relatively case of thehigh Al2O the were degree of sintering of the Al2O3 of sintering ofsintered the Al2Otogether. 3 particles and open pores distributed independently 3 substrate, particles was the relatively highceramic and open pores However, were distributed throughout the sintered ceramic throughout sintered blocks. in theindependently cases of the ZrO 2 substrate, the open pores blocks. However, in the cases of the ZrO substrate, the open pores were interconnected, and some were interconnected, and some raw ZrO be 2 2 blocks could be observed. The key differences canraw ZrO be observed. The key differences can be summarized as increased, follows: theMgO degree of2O sintering summarized as follows: the degree of sintering densification gradually > Al 3 > Y2O3 2 blocks could densification gradually increased, MgO > Al2O3 > Y2O3 > ZrO2. > ZrO2.

Figure 2. Microstructures before sessile drop experiments: Figure Microstructuresof ofthe thevarious variousoxide oxideceramic ceramicsubstrates substrates before sessile drop experiments: (a) MgO; MgO; (b) (b) YY22O33; (c) Al22O (a) O33; ;and and(d) (d)ZrO ZrO2.2 .

Figure 3 shows the microstructures of untreated substrates in contact with the alloys near the Figure 3 shows the microstructures of untreated substrates in contact with the alloys near the three-phase line, posterior to the wetting experiments. The ceramic structures contacted with the three-phase line, posterior to the wetting experiments. The ceramic structures contacted with the alloy melts had a liquid-phase contact and re-sintered, and no alloy liquid was penetrated into the alloy melts had a liquid-phase contact and re-sintered, and no alloy liquid was penetrated into the substrates. The four kinds of crucibles have superior corrosion resistance. In the case of the Al2O3 and substrates. The four kinds of crucibles have superior corrosion resistance. In the case of the Al2 O3 ZrO2 systems, Al2O3 and ZrO2 particles re-sintered into larger ceramic blocks, and the open pores and ZrO2 systems, Al2 O3 and ZrO2 particles re-sintered into larger ceramic blocks, and the open became closed pores. In the case of the MgO system, white protrusions were found and EDS analysis pores became closed pores. In the case of the MgO system, white protrusions were found and EDS confirmed that the protrusions were composed of MgO clusters, which was due to the alloy melts analysis confirmed that the protrusions were composed of MgO clusters, which was due to the alloy adhering to the substrate during the wetting process, so that MgO particles on the surface of the melts adhering to the substrate during the wetting process, so that MgO particles on the surface of the substrate were stuck. substrate were stuck.

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(a)(a) MgO; (b)(b) Y2O Figure 3. Microstructures Microstructures of ofthe thevarious variousoxide oxideceramic ceramicsubstrates substratesafter afterwetting: wetting: MgO; Y23;O3 ; (c) Al22O33;; and and (d) (d) ZrO ZrO22. . Figure 3. Microstructures of the various oxide ceramic substrates after wetting: (a) MgO; (b) Y2O3;

3.2. Wetting Behavior of(d) theZrO Alloys on the the Oxide Oxide Ceramics Ceramics Substrates Substrates 3.2. Wetting(c) Behavior of the Alloys on Al2O3; and 2.

Figures 4 andBehavior 5 show theAlloys changes contact angle of the drop during the continuous continuous time time rise, rise, 3.2. Wetting of the on thein Oxide Ceramics Substrates thethe experiment time, the the contact angles of molten alloysalloys on MgO, Y2O3, at 1873 1873 K. K.With Withthe theincrease increaseofof experiment time, contact angles of molten on MgO, Figures 4 and 5 show the changes in contact angle of the drop during the continuous time rise, ◦ As evident Al2 O 2O3,, Al 2 substrates rapidlyrapidly becamebecame stable and was larger 140°. As evident from Figure Y OZrO ZrO substrates and was than larger than 140 3 and 3 , and at21873 K. With the2 increase of the experiment time, stable the contact angles of molten alloys on .MgO, Y2O3, from 5, alloys had a similar wettability on all of these substrates with initial contact angles in the range of Figure 5, similar wettability on stable all ofand these initial contact angles in the Al2alloys O3, andhad ZrO2asubstrates rapidly became wassubstrates larger than with 140°. As evident from Figure ◦ ◦ ◦ ◦ 143° toof157° and equilibrium contact on angles 20 min) 140° andangles 157°.and the case 5,143 alloys a similar wettability all of (after these substrates with initial contact inInthe range range tohad 157 and equilibrium contact angles (afterbetween 20 min) between 140 157 . Inofof theMgO, case ◦ ◦ ◦ 143° to 157° and equilibrium contact angles (after 20 min) between 140° and 157°. In the case of MgO, Y 2 O 3 , Al 2 O 3 , and ZrO 2 substrates, the equilibrium angles were 140°, 148°, 154°, and 157°, respectively. of MgO, Y2 O3 , Al2 O3 , and ZrO2 substrates, the equilibrium angles were 140 , 148 , 154 , and 157◦ , Y2O3, no Al2Ocharacteristic 3, and ZrO2 substrates, the equilibrium angles were 140°, 148°, 154°, and 157°, respectively. There was transition intransition wettability and all theand systems exhibited wetting respectively. There was no characteristic in wettability all the that systems that exhibited There was no characteristic transition in wettability and all the systems that exhibited wetting behaviorbehavior were found tofound be non-wetting when the typethe of substrates changed.changed. This indicated that the wetting were to be non-wetting when type of substrates This indicated behavior were found to be non-wetting when the type of substrates changed. This indicated that the Ni-20Co-20Cr-10Al-1.5Y alloys and the four different oxide ceramics were non-wetting. that theNi-20Co-20Cr-10Al-1.5Y Ni-20Co-20Cr-10Al-1.5Y and four different oxidewere ceramics were non-wetting. alloysalloys and the fourthe different oxide ceramics non-wetting.

Figure 4. Wetting process of the Ni-20Co-20Cr-10Al-1.5Y alloys on various ceramic substrates

Figure 4. Wetting process of the Ni-20Co-20Cr-10Al-1.5Y alloys on various ceramic substrates substrates substrates at 1873 K. at 1873 K.

Figure 4. Wetting process of the Ni-20Co-20Cr-10Al-1.5Y alloys on various ceramic substrates substrates at 1873 K.

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Figure 5. Variation in the apparent contact angle with time with regard to the Ni-20Co-20Cr-10Al-

Figure 5. Variation in the apparent contact angle with time with regard to the Ni-20Co-20Cr-10Al-1.5Y 1.5Y alloys on the various oxide ceramic substrates at 1873 K. alloys on the various oxide ceramic substrates at 1873 K. Figure 5. Variation the apparent contact with time with regard to the Ni-20Co-20Cr-10Al3.3. Interfacial Reactionsin between the Alloys and angle Ceramics 1.5Y alloys on thebetween various oxide ceramicand substrates at 1873 K. 3.3. Interfacial Reactions the Alloys Ceramics

The typical SEM images of the alloy/ceramic interfaces, sectioned perpendicular to the interfaces

of the four non-wetting systems, are shown Figure 6. It can be seen thatperpendicular the interface was and The typical SEM images of the alloy/ceramic interfaces, sectioned to clean the interfaces 3.3. Interfacial Reactions between the Alloys andinCeramics were no sand adhesion and reaction layers for the6.Y2O 3 can system (Figure 6b).the However, an was of thethere four non-wetting systems, are shown in Figure It be seen that interface The typical SEM images of the alloy/ceramic interfaces, sectioned perpendicular to the interfaces approximately 25 μm continuous layer of white product were observed at the interface for the MgO, cleanofand there were nosystems, sand adhesion reaction layers for the (Figure 2 O3 system the four non-wetting are shown and in Figure 6. It can be seen that theYinterface was clean and 6b). Al2O3 and ZrO2 systems (Figure 6a,c,d). EDS analysis indicated that Y and O are the two dominant However, approximately 25 µm continuous layerfor of white product observed at the interface there an were no sand adhesion and reaction layers the Y2O 3 systemwere (Figure 6b). However, an elements, which suggested that this layer mainly corresponded to Y2O3, and XRD analysis further for the MgO, Al O and ZrO systems (Figure 6a,c,d). EDS analysis indicated that Y and O are the approximately 25 layer of white product were observed at the interface for the MgO, 3 μm continuous confirmed it2 (Figure 7). It 2was indicated that interfacial reactions between MgO, Al2O3, and ZrO2 Al2O3 and ZrO 2 systems (Figure 6a,c,d). EDS analysis indicated thatcorresponded Y and O are theto two two dominant which suggested that this layer mainly Y2dominant O3 ,toand substrates, elements, and Ni-20Co-20Cr-10Al-1.5Y alloys occurred at high temperature. In addition the XRD elements, which suggested that this layer mainly corresponded to Y2Oreactions 3, and XRD analysisMgO, further analysis further confirmed it (Figure 7). It was indicated that interfacial between Al2 O3 , interface reaction layer, a sand adhesion layer was also observed at the interface of the Al 2O3 system confirmed it (Figure 7). It was indicated that interfacial reactions at between MgO, Al2O3, and ZrO2 to and ZrO substrates, and Ni-20Co-20Cr-10Al-1.5Y alloys occurred high temperature. In addition (Figure 6c). EDS analysis indicated that Al and O were the two dominant elements, which suggested 2 substrates, and Ni-20Co-20Cr-10Al-1.5Y alloys occurred at high temperature. In addition to the the interface layer,layer a sand adhesion layer was also at the interface of the Al2was O3 system that the reaction sand adhesion was Al2O3 stripped from theobserved ceramic surface. Interestingly, there a interface reaction layer, a sand adhesion layer was also observed at the interface of the Al 2O3 system thin6c). layer of analysis Y2O3 solidindicated solution between the reaction and dominant the sand adhesion layer, forming a (Figure EDS that Al and O werelayer the two elements, which suggested (Figure 6c). EDS analysis indicated that Al and O were the two dominant elements, which suggested sandwich-like structure as shown image of from the alloy Al2O3 system. that the sand adhesion layer was AlbyOthestripped theofceramic surface. Interestingly, there was that the sand adhesion layer was Al2 2O33 stripped from the ceramic surface. Interestingly, there was a a thinthin layer of Y O solid solution between the reaction layer and the adhesion forming 2 3 layer of Y2O3 solid solution between the reaction layer and the sandsand adhesion layer, layer, forming a a sandwich-like structure as shown by the image of the alloy of Al O system. 3 sandwich-like structure as shown by the image of the alloy of Al2O32system.

Figure 6. Morphologies of the metal/substrate interfaces: (a) MgO; (b) Y2O3; (c) Al2O3; and (d) ZrO2.

6. Morphologies of the metal/substrateinterfaces: interfaces: (a) ; (c) Al2O3; and (d) ZrO2. FigureFigure 6. Morphologies of the metal/substrate (a)MgO; MgO;(b) (b)Y2YO23O 3 ; (c) Al2 O3 ; and (d) ZrO2 .

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Figure EDS the substrate interface. Figure7. EDSand andmicro-XRD micro-XRDpatterns patternsof themetal/MgO metal/MgOsubstrate substrateinterface. interface. Figure 7.7.EDS and micro-XRD patterns ofofthe metal/MgO

Imagesofofthe thecross-sections cross-sectionsofofthe thealloys alloysnear nearthe thetriple tripleline lineand andthe thealloy alloymatrix matrixare areshown showninin Images Figure8.8.8. The Ni-20Co-20Cr-10Al-1.5Y alloys had the same phase composition composition as the the cast cast Figure The Ni-20Co-20Cr-10Al-1.5Y alloysalloys had thehad samethe phase composition as the cast microstructure The Ni-20Co-20Cr-10Al-1.5Y same phase as microstructure on the four different substrates, including the γ phase (fcc structural matrix phase), γ’ on the four different substrates, including the γ phase (fcc structural matrix phase), γ’ phase (Ni Al), microstructure on the four different substrates, including the γ phase (fcc structural matrix phase), 3 γ’ phase Al1414Co Co3Ni 3Ni and Ni 3Yphase, phase, asshown shown Figure 9.Some Someoxide oxide particles were Al Ni3Al), phase, and Ni Y phase, asNi shown in Figure 9. Some oxide9.particles wereparticles also found in phase Al 3 3phase, and 3Y as ininFigure were 14 Co(Ni 3(Ni 33Al), 3phase, also found thealloy alloy matricesofof allarea systems. Thearea area percentages theoxide oxide inclusions, basedon on the alloy matrices of allmatrices systems. The percentages ofpercentages the oxide inclusions, based on the images of also found ininthe all systems. The ofofthe inclusions, based the images of the longitudinal sections, are shown in Tables 2 and 3. The alloy of the MgO system the longitudinal are shown in Tables 2 and in 3. Tables The alloy of the MgOalloy system hadMgO the greatest images of thesections, longitudinal sections, are shown 2 and 3. The of the system hadthe thegreatest greatest inclusions content, while the 2the O3 3system system had thelowest lowest content.area The inclusions content, while the alloy of the the Ythe system had lowesthad content. The average had inclusions content, while ofofthe YY2O the content. The 2 Oalloy 3alloy average area percentage of the oxide particles of the four systems were 0.59% (MgO), 0.11% (Al 2 O23 3),), percentage of the oxide particles of the four systems were 0.59% (MgO), 0.11% (Al O ), 0.09% (ZrO average area percentage of the oxide particles of the four systems were 0.59% (MgO), 2 3 0.11% (Al2O 0.09% (ZrO 2 ), and 0.02% (Y 2 O 3 ), respectively. and 0.02% O3 ), 0.02% respectively. 0.09% (ZrO(Y 2),2 and (Y2O3), respectively.

Figure Themorphologies morphologiesof the alloy matrices:(a) (a) MgO; (b) 2O 3; (c) Al2O3; and(d) (d)ZrO ZrO2.2.. Figure The morphologies ofof the alloy matrices: (a) MgO; (b) O Figure 8.8.8. The the alloy matrices: MgO; (b) YY2Y2O and (d) ZrO 33; (c) Al22O33;; and 2

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Figure 9. X-ray microdiffraction patterns patterns of of the the alloys alloys of of the the Y Y2O 3 systems. Figure 9. X-ray microdiffraction 2 O3 systems. Table 2. Chemical compositions obtained via EDS analysis (at%). Table 2. Chemical compositions obtained via EDS analysis (at%).

Element O Al Point 1 Element 8.52 O 23.68Al 1 8.52 37.71 23.68 Point 2 Point 2.29 Point 2 2.29 Point 3 2.44 7.0537.71 Point 3 2.44 7.05 Point 4 Point12.60 4 12.605.675.67

Y Y 0.45 0.45 0.35 0.35 0.21 0.21 2.76 2.76

Cr 9.51 9.51 9.19 9.19 26.74 26.74 14.31 14.31 Cr

Co 11.74 Ni 11.74 10.8846.10 10.88 21.3839.58 21.38 42.17 11.9930.57 11.99 Co

Ni Zr 46.10 39.58 42.17 30.57 22.10

Zr 22.10

Table 3. Oxide inclusion contents of the alloys, based on the images of the longitudinal sections of the Table 3. Oxide inclusion contents of the alloys, based on the images of the longitudinal sections of the alloys (excluding the reaction layer). alloys (excluding the reaction layer). Number of Oxide Average Diameter of Oxide Area Percentage of Systems Number of Average Diameter 3of Area Percentage of Particles Particles/μm Oxide Inclusion Systems 3 Oxide Oxide Inclusion MgO 57 Particles Oxide Particles/µm 31.3 0.59% Al2O3 MgO 125 57 6.8 31.3 0.59% 0.11% 6.8 0.11% 0.09% ZrO2 Al2 O3 64 125 10.1 10.1 0.09% 0.02% Y2O3 ZrO2 42 64 3.9 Y2 O3 42 3.9 0.02%

4. Discussion 4. Discussion 4.1. Wetting Kinetics and Driving Force 4.1. Wetting Kinetics and Driving Force In the four systems, the contact angles were stable during the melting process of alloys and the In the four systems, the contact angles stable melting non-wetting process of alloys and the equilibrium contact angles were larger thanwere 90°. All theduring systemsthe exhibited behavior. In ◦ . All the systems exhibited non-wetting behavior. equilibrium contact angles were larger than 90 order to try to explain the changing in the wetting characteristics as a function of both alloys and In order to try to explain changing in the characteristics as a function of both and substrate composition, thethe surface tension andwetting the roughness surface were carried out on allalloys samples. substrate composition, theofsurface tension the roughness surfacethermophysical were carried out on all samples. The surface tension molten metalsand is the most important property which The surface tension of molten metals is the most important thermophysical property which dominates thermo and hydrodynamic processes at the surface, the surface tension value is larger, dominates thermo and hydrodynamic processes at the surface, the surface tension value is larger, and the spread of alloy melts is more difficult [26]. The solid/liquid surface tensions for substrates and the spread of meltsthe is more difficult of [26]. solid/liquid surfaceequation: tensions for substrates were calculated byalloy analyzing photographs theThe droplet using Young’s were calculated by analyzing the photographs of the droplet using Young’s equation:  lv cos   sv   sl (1) γlv cos θ = γsv − γsl (1) WA   lv (1  cos  ) (2) WA = γlv (1 + cos θ ) (2) The variation of the surface tension as a function of time on the different substrates is shown in Figure 10. The surface tension in the present work was much higher than 670–700 mN/m. And the average surface tension with respect to the type of the ceramic substrates gradually increased, ZrO2 > Al2O3 > Y2O3 > MgO.

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The variation of the surface tension as a function of time on the different substrates is shown in Figure 10. The surface tension in the present work was much higher than 670–700 mN/m. And the average surface tension with respect to the type of the ceramic substrates gradually increased, ZrO2 > 2018, Al2 O11, Y2 OPEER 3> 3 > MgO. Materials x FOR REVIEW 9 of 13 Materials 2018, 11, x FOR PEER REVIEW

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Figure 10. Variation in the surface tension with time with regard to the wetting of the molten alloys Figure 10. Variation in the surface tension with time with regard to the wetting of the molten alloys on Figure 10. Variation in theatsurface tension with time with regard to the wetting of the molten alloys on the different substrates 1873 the different substrates at 1873 K. K. on the different substrates at 1873 K.

A schematic representation of the molten alloys from the 1st to 2nd position is shown in Figure A representation ofofthe molten thethe 1st1st to 2nd position is shown in Figure 11b. A schematic schematic representation the molten from towas 2nd position is shown in Figure 11b. Due to the surface tension (Po), the liquidalloys atalloys thefrom curved surface different from the plane and Due to the surface tension (Po), the liquid at at the curved surface was from and 11b.liquid Due to the surface tension (Po), the liquid the curved surface wasdifferent different fromthe the plane plane(Ps), and the molecules at the surface were always subjected to an additional contraction pressure the liquid molecules at the surface were always subjected to an additional contraction pressure the liquidtomolecules always subjected contraction pressure (Ps), pointing the centeratofthe thesurface sphere.were Under the action of Pto(Pan = additional Po + Ps), the melt alloys had a non(Ps), pointing tocenter the center ofsphere. the sphere. Under the action of=PPo (P+=Ps), Po the + Ps), thealloys melt had alloys had pointing to the of the Under the action of P (P melt a nonwetting displacement without spreading on the crucible wall, as shown in Figure 11. awetting non-wetting displacement without spreading on the crucible wall, as shown in Figure 11. displacement without spreading on the crucible wall, as shown in Figure 11.

Figure 11. Schematic configurations of a sessile drop in the case of θ > 90°: (a) at the solid-liquid-vapor ◦ : (a) Figureline; 11. and Schematic configurations of aa sessile sessile drop dropin inthe the case case of of θθ>> 90 90°: triple (b) from 1st to 2nd position. Figure 11. Schematic configurations of (a) at at the the solid-liquid-vapor solid-liquid-vapor triple line; and (b) from 1st to 2nd position. triple line; and (b) from 1st to 2nd position.

The influence of substrate surface roughness on the wetting behavior could be attributed to the The influence of line substrate surface roughness the wettingtobehavior could be the pinning the triple by sharp edges that acton as spreading of attributed theattributed liquidto[27]. Theofinfluence of substrate surface roughness onobstacles the wettingthe behavior could be to pinning oftothe triple [28] lineand by sharp edges that act asinobstacles toofthe spreading of the liquid [27]. According Wenzel Cassie and Baxter [29], the region wettability (θ < 90°), roughness the pinning of the triple line by sharp edges that act as obstacles to the spreading of the liquid [27]. According to Wenzel [28] and Cassie and Baxter [29], in the region of wettability (θ < 90°), roughness must causeto the contact angle decrease comparison with the contact angle on surface. According Wenzel [28] andto Cassie and in Baxter [29], in the region of wettability (θa 90°), the equilibrium angle on the rough substrate surface would must cause the contact angle to decrease in comparison with the contact angle on a smooth surface. In the system of non-wettability (θ > 90°), the equilibrium angle on the rough substrate surface would be higher, thatofis, θ’ > θ. The rough interface reduced contact area the molten alloys and ◦ ), the equilibrium In the system non-wettability (θ > 90 angle on thebetween rough substrate surface would be higher, that is, θ’ > θ. The rough interface reduced contact area between the molten alloys and substrate as shown in Figure 12. substrate as shown in Figure 12.

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be higher, that is, θ’ > θ. The rough interface reduced contact area between the molten alloys and substrate as shown Figure 12. Materials 2018, 11, x FORinPEER REVIEW 10 of 13

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Figure12. 12.Schematic Schematicconfigurations configurations of of aasessile sessiledrop dropresting restingon on the the substrate substratewith withrough rough surfaces surfaces in in Figure Figure 12. Schematic configurations of a sessile drop resting on the substrate with rough surfaces in ◦ the case of θ > 90°. the case case of of θ θ> 90 . the > 90°.

In this study, in in any any case, case, the the contact contact angle angle of of the the alloy/ceramic alloy/ceramic system was larger than 140° 140° ◦ In this study, in any case, the contact angle of the alloy/ceramic system was larger than 140 within the measuring time. The relationship between the surface line roughness values and the levels within the measuring time. The relationship between the surface line roughness values and the levels of open open porosity porosity of of the the ceramic ceramic substrates substrates isis shown shown in in Figure Figure 13. 13. There There was was aa positive positive correlation correlation of betweenthe thesurface surfaceroughness roughnessvalues valuesand andthe thelevels levelsof of openporosity. porosity. According to the experimental between of open open porosity. According to the experimental results in Figure 13, in the case of the substrates with open porositylevels levels of of 11.8% 11.8%(MgO), (MgO), 20.44% results in Figure 13, in the case of the substrates with open porosity (MgO), 20.44% 20.44% (Y 2O3), 24.41% (Al2O3), and 26.42% (ZrO2), the surface roughness values, Ra, were determined to be (Y22O O33),), 24.41% 24.41% (Al (Al22OO3), and26.42% 26.42%(ZrO (ZrO2), thesurface surfaceroughness roughnessvalues, values,Ra, Ra, were were determined determined to be 3 ),and 2 ),the 2.127μm μm (MgO), 5.886μm μm (Y (Y2O3), 7.856μm μm (Al22O O3), and 15.44 15.44 μm(ZrO (ZrO22),), respectively. respectively. The surface 2.127 µm (MgO), 5.886 µm (Y22O33), 7.856 µm (Al2 33),), and and 15.44 μm µm 2 ), respectively. The surface roughness of the four different oxide substrates gradually increased and these surface structureswere were roughness of the four different different oxide substrates gradually increased and these these surface surface structures structures more complicated. The pinning of the triple line by any sharp edges could also obstruct the spreading more complicated. The pinning of the triple line by by any any sharp sharp edges edges could could also also obstruct obstruct the the spreading spreading ofthe theliquid. liquid.Thus, Thus,the thecontact contactangle anglewould wouldincrease increase asthe the surface roughnessincreased. increased. of as surface roughness increased.

Figure 13. The relationship between the surface roughness values (Ra) and the levels of open porosity Figure 13. The relationship between the surface roughness values (Ra) and the levels of open porosity Figure 13. The oxide relationship between the surface roughness values (Ra) and the levels of open porosity of the various ceramics. of the various oxide ceramics. of the various oxide ceramics.

4.2.Thermodynamic ThermodynamicAnalysis AnalysisofofInterfacial InterfacialReactions Reactions 4.2. Inthe thefour fournon-wetting non-wettingsystems, systems,YYsegregated segregatedat atthe theinterface, interface,and andthen thenprecipitated precipitatedas asaareaction reaction In product, YY22O O33.. According According to to the the Gibbs Gibbs free free energy energy of of formation formation [30], [30], shown shown in in Table Table 4,4, the the product, thermodynamic stability of the refractory can be described by the following sequence: Y 2O3 > ZrO2 > thermodynamic stability of the refractory can be described by the following sequence: Y2O3 > ZrO2 > Al22O O33 >> MgO. MgO. Saha Saha proposed proposed that, that, when when there there was was little little variation variation in in the the free free energy, energy, the the Al contamination of the alloy matrix owed to refractory oxides may not just be related to the contamination of the alloy matrix owed to refractory oxides may not just be related to the thermodynamic stability stability of of the the oxide, oxide, but but may may also also be be influenced influenced by by the the solution solution effect effect [31]. [31]. thermodynamic

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4.2. Thermodynamic Analysis of Interfacial Reactions In the four non-wetting systems, Y segregated at the interface, and then precipitated as a reaction product, Y2O3. According to the Gibbs free energy of formation [30], shown in Table 4, the thermodynamic stability of the refractory can be described by the following sequence: Y2O3 > ZrO2 > Al2O3 > MgO. Saha proposed that, when there was little variation in the free energy, the contamination of the alloy matrix owed to refractory oxides may not just be related to the thermodynamic stability of the oxide, but may also be influenced by the solution effect [31]. Therefore, the occurrence of the interfacial reactions in the alloy/ceramic substrate could be ascribed to the thermal dissociation of MgO, Al2O3, and ZrO2, which was shown in Table 4: Table 4. Gibbs free energy of formation of the metallic oxides at 1900 K. Oxides

∆Gf (kJ/mol)

No.

MgO(s) = Mg(l) + O(g) 1/3Al2 O3 (s) = 2/3Al(l) + O(g) 1/2ZrO2 (s) = 1/2Zr(l) + O(g) 1/3Y2 O3 (s) = 2/3Y(l) + O(g) Al2 O3 (s) ↔ Al2 O(g) + O(g)

339.741 355.326 372.979 454.243 -

(3) (4) (5) (6) (7)

In the three systems, sufficient [O] atoms were dissolved during the reversible reaction, and the Y tended to segregate at the alloy/ceramic interface. Thus, the released [O] atoms could combine with Y to form Y2 O3 . The melting point of Y2 O3 (1873 K) was greater than that of MgO, Al2 O3 , and ZrO2 (1873 K), and the value of free energy change associated with reactions (3), (4), and (5) should be lower than that of reaction (6). Therefore, the Y2 O3 reaction layer should precipitate (reaction (6)) at the interface. When the diffusion in the molten alloys was more rapid than the interfacial reaction, the spreading was limited by the interfacial reaction. However, if the growth of the reaction product at the triple line was limited by the transmission of reaction elements, the spreading was limited by diffusion [32]. In the case of the MgO system, the fine MgO particles were sintered together, which indicated that thermal dissociation occurred at the interface and within the substrate. When the [O] atoms at the interface were consumed by Y to form Y2 O3 , the [O] atoms located far from the interface could easily travel to the interface because of the proliferation of the molecules at high temperature. However, a reaction layer had formed at the interface. Therefore, the new [O] atoms would diffuse into the molten alloys owing to the chemical potential gradient of [O] between the MgO substrate and the molten alloys. When the [O] content reached a certain limit, oxide inclusions would precipitate within the alloy matrix. In the other two systems, owing to the low degree of sintering and the presence of the raw, inactive Al2 O3 and ZrO2 particles at the surface, compared with the MgO substrate, there were fewer [O] atoms released. When the molten alloys were dropped, there were insufficient [O] atoms present to react with the Y to form Y2 O3 immediately. Therefore, the reaction layer was irregular compared to that of the MgO system. When the [O] atoms located far from the interface moved to the interface, the formed reaction layer prevented contact between the Y and [O]. Therefore, the reaction layer could not increase, and fewer [O] atoms would diffuse into the alloys. It is noteworthy that the Al2 O3 and ZrO2 system possessed the lower oxide inclusion content. This could be attributed to the surface structure. In the case of the MgO system, the MgO substrate with lower thermal stability should surely lead to high oxide inclusions. In the case of the Al2 O3 and ZrO2 systems, substrates with higher thermal stability and low degree of sintering resulted in a weak reaction. Moreover, in the case of the Al2 O3 system, Y2 O3 and Al2 O3 had the same lattice structure, and they could form a continuous solid solution at a certain temperature. A thin layer of Y2 O3 solid solution between the reaction layer and the sand adhesion layer formed.

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Based on the analysis above, the wetting behavior and interactions between Ni-20Co-20Cr-10Al-1.5Y alloys and various oxide ceramics were affected by the existence of the Y2O3 layer at the interface. The alloys after reacting with the MgO ceramic had the highest inclusion content, while those of the lowest content were in the Y2O3 system. This indicated that Y2O3 ceramic was most beneficial with regard to the vacuum induction melting of high-purity Ni-20Co-20Cr-10Al-1.5Y alloys. 5. Conclusions The wetting of molten Ni-20Co-20Cr-10Al-1.5Y alloys on MgO, Y2 O3 , Al2 O3 , and ZrO2 ceramic substrates were studied by an improved sessile-drop method at 1873 K. Based on which the high-temperature wettability, interactions, and associated mechanisms were discussed. The primary conclusions of this work are as follows:

•

•

•

•

With an increase of the experiment time, the contact angles of molten alloys on MgO, Y2 O3 , Al2 O3 , and ZrO2 substrates rapidly became stable. The equilibrium angles were 140◦ , 148◦ , 154◦ , and 157◦ , respectively. There was no characteristic transition in wettability when the type of substrates changed and no alloy liquid penetrated into the substrates. The influence of different ceramics on the wettability can be explained by the surface tension of the melts and the surface roughness of the substrates. With the exception of the Y2 O3 system, an approximately 25 µm continuous Y2 O3 reaction layer was observed along the interface of the alloys. The Y2 O3 layer significantly reduced the wettability and interactions of Ni-20Co-20Cr-10Al-1.5Y alloys on MgO, Al2 O3 , and ZrO2 . Oxide particles were found in the alloy matrices of all the systems. The average area percentage of oxides were 0.59% (MgO), 0.11% (Al2O3), 0.09% (ZrO2), and 0.02% (Y2O3), respectively. The formation of oxides could be attributed to the thermal dissociation of substrates, which provided sufficient [O] atoms that could dissolve into the molten alloys easily. The Y2 O3 ceramic with open porosity levels of 20.44% was most beneficial with regard to the vacuum induction melting of high-purity Ni-20Co-20Cr-10Al-1.5Y alloys.

Author Contributions: Hua.Z. and Hu.Z. conceived and designed the experiments; T.T. and M.G. performed the experiments; J.L. analyzed the data; Q.L. and W.B. contributed reagents/materials/analysis tools; and J.L. and Hua.Z. wrote the paper. Funding: This work was supported by the National Science and Technology Pillar Program of China (no. 2013BAB11B04) and the National Natural Science Foundation of China (grant no. 51404017 and no. 51604014). Acknowledgments: The authors wish to express their appreciations to the State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology. Conflicts of Interest: The authors declare no conflict of interest.

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