Microstructure and Properties of Porous Si3n4/Dense Si3n4 Joints Bonded Using RE–Si–Al–O–N (RE = Y Or Yb) Glasses

metals

Article Microstructure and Properties of Porous Si3N4/Dense Si3N4 Joints Bonded Using RE–Si–Al–O–N (RE = Y or Yb) Glasses

Ling Li, Liangbo Sun ID , Chunfeng Liu *, Xinhua Wang, Xuanzhi Wang and Jie Zhang *

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China; [email protected] (L.L.); [email protected] (L.S.); [email protected] (X.W.); [email protected] (X.W.) * Correspondence: [email protected] (C.L.); [email protected] (J.Z.); Tel./Fax.: +86-0451-8641-4234 (J.Z.)

Received: 15 October 2017; Accepted: 9 November 2017; Published: 13 November 2017

Abstract: The joining of porous Si3N4 to dense Si3N4 ceramics has been successfully performed using mixed RE2O3 (RE = Y or Yb), Al2O3, SiO2, and α-Si3N4 powders. The results suggested that the α-Si3N4 powders partly transformed into β-SiAlON and partly dissolved into oxide glass to form oxynitride glass. Thus, composites of glass/β-SiAlON-ceramic formed in the seam of joints. Due to the capillary action of the porous Si3N4 ceramic, the molten glass solder inﬁltrated into the porous Si3N4 ceramic side during the joining process and formed the “inﬁltration zone” with a thickness of about 400 µm, which contributed to the heterogeneous distribution of the RE–Si–Al–O–N glasses in the porous Si3N4 substrate. In-situ formation of β-SiAlON in the seam resulted in a high bonding strength. The maximum bending strength of 103 MPa and 88 MPa was reached for the porous Si3N4/dense Si3N4 joints using Y–Si–Al–O–N and Yb–Si–Al–O–N glass solders, respectively.

Keywords: silicon nitride; glass solder; β-SiAlON; joining; inﬁltration

1. Introduction

Dense silicon nitride (D-Si3N4) ceramics are considered to be important structure materials applicable for various industry parts [1] for its superior high temperature strength, thermal shock resistance, low thermal expansion coefficient, good wear, and corrosion resistance [2–4]. The porous Si3N4 (P-Si3N4) ceramics are emphasized for some engineering applications such as catalyst supports and gas filters due to its high mechanical strength, low dielectric constant, low density, and porous nature [5–8]. In the present work, the porous Si3N4 ceramics were tried to join to the dense Si3N4 to combine the performance advantages of both the materials. In order to widen their applications, reliable joining techniques are essential, among which the brazing and diffusion bonding techniques are especially popular for the ceramics [9,10]. However, large residual stress inevitably resided in the joint after brazing or diffusion bonding due to the mismatch of the coefficient of thermal expansion (CTE) between the metallic layers and the ceramic materials [11–13]. Currently, the bonding of ceramics with oxide or oxynitride glass adhesives has been successful [14–17]. Since the sintering aids contribute to excellent bonding between Si3N4 grains during the synthesis process, they should also be applicable for the joining of bulk materials. In the dense Si3N4 ceramics, conventional sintering aids such as Y2O3, Al2O3, MgO, and CaO have been widely used as the joint interlayers [18–20]. Walls et al. [21,22] used composite β-Sialon-glass adhesives (a mixture of Y2O3, SiO2, Al2O3, and α-Si3N4) to join Sialon ceramics. Their results suggest that the α-Si3N4 reacted to form β-Sialon, which had an acicular nature to reinforce the joint. However, these studies have all focused on the joining of dense silicon nitride ceramics.

Metals 2017, 7, 500; doi:10.3390/met7110500 www.mdpi.com/journal/metals Metals 2017, 7, 500 2 of 9

In the present work, porous Si3N4 ceramic was firstly bonded to dense Si3N4 using RE (Y2O3 or Yb2O3)-AlMetals2O 20173-SiO, 7, 5002-Si 3N4 mixtures, which in-situ formed a high proportion of β-SiAlON in2 of the 9 seam; the influence of bonding parameters and solder composition on the interfacial morphology and joint In the present work, porous Si3N4 ceramic was firstly bonded to dense Si3N4 using RE (Y2O3 or strengths were studied. In addition, the influence of oxynitride glasses on the microstructure of Si N Yb2O3)-Al2O3-SiO2-Si3N4 mixtures, which in-situ formed a high proportion of β-SiAlON in the seam; 3 4 substratesthe wasinfluence also of investigated. bonding parameters Finally, and the solder infiltration composition behavior on the of interfacial the glass morphology filler in the and porous joint Si3N4 substratestrengths was revealed. were studied. In addition, the influence of oxynitride glasses on the microstructure of Si3N4 substrates was also investigated. Finally, the infiltration behavior of the glass filler in the porous 2. MaterialsSi3N4 substrate and Methods was revealed.

The2. porousMaterials Si and3N4 Methodsceramics (Shandong Industrial Ceramics Research & Design Institute Co., Ltd., Zibo, China) selected in this study with a porosity of approximately 47% were prepared via gas The porous Si3N4 ceramics (Shandong Industrial Ceramics Research & Design Institute Co., pressureLtd., sintering Zibo, China) (GPS). selected The dense in this Sistudy3N4 withceramics a porosity (Shandong of approximately Industrial 47% ceramics were prepared Research via gas & Design Institutepressure Co., Ltd., sintering Zibo, (GPS). China) The were dense pressurelessly Si3N4 ceramics sintered(Shandong and Industrial contained ceramics a small Research percentage & of Al2O3 andDesign Y2 OInstitute3 as sintering Co., Ltd., aids. Zibo, The China) four-point were bendingpressurelessly strengths sintered of and the porouscontained Si 3aN small4 and dense percentage of Al2O3 and Y2O3 as sintering aids. The four-point bending strengths of the porous Si3N4 Si3N4 were 142 MPa and 486 MPa, respectively. Figure1a shows the morphology of the porous Si 3N4 and dense Si3N4 were 142 MPa and 486 MPa, respectively. Figure 1a shows the morphology of the substrate with elongated grains, which were randomly connected in three dimensions. porous Si3N4 substrate with elongated grains, which were randomly connected in three dimensions.

Figure 1. Characterization of the porous Si3N4 ceramic (a) and test jig geometry used for four-point Figure 1. Characterization of the porous Si N ceramic (a) and test jig geometry used for four-point bend testing (b). 3 4 bend testing (b). The raw ceramics ingots were cut into blocks with the dimensions of 4 mm × 4 mm × 4 mm to be Thejoined raw for ceramics microstructural ingots wereanalysis, cut and into 3 blocksmm × 4 with mm × the 18 dimensionsmm to be joined of4 for mm flexural× 4 mmstrength× 4 mm to testing. The working surfaces were ground using waterproof abrasive 1000 grit paper, followed by a be joined for microstructural analysis, and 3 mm × 4 mm × 18 mm to be joined for flexural strength mechanical polish to 1 μm. testing. TheThe working compositions surfaces of the were glass ground solders using (Y–Si–Al–O–N waterproof and abrasiveYb–Si–Al–O–N 1000 gritglasses) paper, adopted followed in by a mechanicalthe present polish work to 1 areµm. listed in Table 1. The powders were manually ground in ethanol for 3 h using Thean compositions agate mortar to ofobtain the a glass homogeneous solders slurry. (Y–Si–Al–O–N Afterwards, and the slurry Yb–Si–Al–O–N was dried, the glasses) mixture adoptedwas in the presentcold-pressed work are under listed 6 MPa in to Table obtain1. Thea slice powders of 600 μm werein thickness manually using grounda tablet machine. in ethanol The porous for 3 h using an agateSi mortar3N4 and todense obtain Si3N a4 homogeneousrectangular specimens slurry. together Afterwards, with a thesolder slurry slice was were dried, assembled the mixture in a was sandwich structure. The assembly was placed under a modest uniaxial pressure of 0.6 MPa to ensure cold-pressed under 6 MPa to obtain a slice of 600 µm in thickness using a tablet machine. The porous good contact between the solder and silicon nitride ceramics during the joining process. The joining Si3N4 andexperiment dense Si was3N4 conductedrectangular in a specimens graphite furnace together with witha wide a solderrange of slice bonding were temperatures assembled in(1550– a sandwich structure.1650 The °C) assembly for 30 min. was The placed pressure under of N a2 atmosphere modest uniaxial was kept pressure at 0.04 ofMPa 0.6 throughout MPa to ensure the joining good contact betweenprocess. the solder Four-point and siliconbending nitride strengths ceramics were measured during with the an joining upper process.span of 15 The mm joining and a lower experiment was conductedspan of 30 in mm a graphiteusing a universal furnace testing with machine a wide (Instron range 5569, of bonding Instron Corporation, temperatures Canton, (1550–1650 MA, ◦C) USA) at a speed of 0.5 mm/min. The test jig geometry used for bend testing is shown in Figure 1b. for 30 min. The pressure of N atmosphere was kept at 0.04 MPa throughout the joining process. For each set of experimental2 data, five samples were used to average to joint strength. Scanning Four-pointelectron bending microscope strengths (SEM) were images measured were collecte with and upperwith a spanFEI Quanta of 15 mm 200FEG and ainstrument lower span (FEI of 30 mm using aCorporation, universal testing Hillsporo, machine OR, USA) (Instron at accelerating 5569, Instron voltages Corporation, of 20 kV with Canton, an energy MA, dispersive USA) at a speed of 0.5 mm/min.spectroscope The (EDS, test OXFORD jig geometry Instruments, used for Oxford, bend UK) testing system. is shown The phases in Figure in the1b. joints For were each set of experimental data, five samples were used to average to joint strength. Scanning electron microscope (SEM) images were collected with a FEI Quanta 200FEG instrument (FEI Corporation, Hillsporo, OR, USA) at accelerating voltages of 20 kV with an energy dispersive spectroscope (EDS, OXFORD Instruments, Oxford, UK) system. The phases in the joints were identified by X-ray diffraction (XRD, Metals 2017, 7, 500 3 of 9

PANalytical B.V., Almelo, Netherlands) with Cu Kα radiation at a scanning step of 0.05◦ and a scanning rate of 5◦/min.

Metals 2017, 7, 500 Table 1. Chemical composition of glass solders (wt %). 3 of 9

identified by X-ray diffraction (XRD, PANalytical B.V., Almelo, Netherlands) with Cu Kα radiation Glass Solders Y2O3 Yb2O3 Al2O3 SiO2 α-Si3N4 at a scanning step of 0.05° and a scanning rate of 5°/min. Y-SiAlON 44.5 - 10.5 10.3 34.7 Yb-SiAlONTable 1. Chemical - composition 58.3 of glass 7.9 solders (wt 7.7 %). 26.1

Glass Solders Y2O3 Yb2O3 Al2O3 SiO2 α-Si3N4 3. Results and DiscussionY-SiAlON 44.5 - 10.5 10.3 34.7 Yb-SiAlON - 58.3 7.9 7.7 26.1 3.1. Characterization of P-Si3N4/RE(Y or Yb)-SiAlON/D-Si3N4 Joints 3. Results and Discussion Figure2 shows the back-scattered SEM images of the joint bonded at 1650 ◦C for 30 min using a

Y-SiAlON3.1. glassCharacterization solder. The of P-Si joint3N4/RE(Y was crack-free,or Yb)-SiAlON/D-Si compact,3N4 Joints and uniform. The joint can be divided into three regions: the infiltration zone (on the porous Si N substrate side), the seam zone, and the diffusion Figure 2 shows the back-scattered SEM images3 of4 the joint bonded at 1650 °C for 30 min using a µ zone (onY-SiAlON the dense glass Si solder.3N4 substrate The joint was side). crack-free, An infiltration compact, and zone uniform. with aThe width joint can of aboutbe divided 450 intom can be observedthree in theregions: porous the Siinfiltration3N4 substrate zone (on in Figurethe porous2a, as Si can3N4 substrate white ribbon-like side), the seam structures zone, and distributed the in the infiltrationdiffusion zone. zone (on In orderthe dense to investigateSi3N4 substrate the side). microstructure An infiltration in zone detail, with thea width microstructures of about 450 μm of Zone I were highlycan be magnified, observed in andthe porous they are Si3N shown4 substrate inFigure in Figure2b, 2a, which as can shows white theribbon-like three behaviors structures of the distributed in the infiltration zone. In order to investigate the microstructure in detail, the molten glass. One is the diffusion into dense Si3N4 along the grain boundary to form a diffusion layer. microstructures of Zone I were highly magnified, and they are shown in Figure 2b, which shows the Another is the infiltration into the porous Si3N4, and the densification of the residual solder in the seam. three behaviors of the molten glass. One is the diffusion into dense Si3N4 along the grain boundary to The ribbon-likeform a diffusion structures layer. wereAnother virtually is the infiltration composed into ofthe a porous black Si needle-like3N4, and the phasedensification and a of white the glass phase inresidual Zone II. solder In addition, in the inseam. order The to ribbon-like be observed, structures the direction were virtually of the yellow composed dashed of framea black (Zone II) is rotatedneedle-like and magnified phase and in a Figure white 2glassc. The phase residual in Zone skeleton II. In addition, structure in order of the to porous be observed, Si 3N 4theceramics can be seendirection between of the theyellow two dashed labeled frame lines. (Zone The II) EDS is rotated results and of themagnified spots markedin Figure in2c.the The figure residual are listed in Tableskeleton2. It can structure be concluded of the porous that Si the3N4 whiteceramics substrate can be seen Phase between A was the two Y-SiAlON labeled lines. glass. The In EDS addition, results of the spots marked in the figure are listed in Table 2. It can be concluded that the white β the needle-likesubstrate phasePhase A was-SiAlON Y-SiAlON grains glass. (Phase In addition, B) with the aneedle-like fine structure phase formedβ-SiAlON in grains the infiltration (Phase B) zone (Figure2withd). a Therefore, fine structure it formed was deduced in the infiltration that the zone molten (Figure Y-SiAlON 2d). Therefore, glass it solderwas deduced infiltrated that the into the pores ofmolten the porous Y-SiAlON Si3N glass4 during solder the infiltrated joining process,into the pores contributed of the porous to the transformationSi3N4 during the ofjoining theoriginal α-Si3N4process,to β-SiAlON contributed in the to infiltrationthe transformation zone. Simultaneously, of the original α-Si the3N residual4 to β-SiAlON glass in solder the infiltration in the seam was turned intozone. compositesSimultaneously, of glass/ the residualβ-SiAlON-ceramic. glass solder in The the αseam- to βwas-phase turned transformation into composites is consistentof glass/β-SiAlON-ceramic. The α- to β-phase transformation is consistent with the findings in other with the findings in other literature about the joining of dense Si3N4-based ceramics [22,23]. literature about the joining of dense Si3N4-based ceramics [22,23].

Figure 2. Backscattered SEM images of (a) the joint between P-Si3N4 and D-Si3N4 using Y-SiAlON Figure 2.glassBackscattered solder at 1650 SEM °C imagesfor 30 min. of ( a()b the) The joint magnified between morphology P-Si3N4 and of Zone D-Si 3I,N (4c)using the magnified Y-SiAlON glass ◦ solder atmorphology 1650 C for of Zone 30 min. II, and (b) (d The) the magnified magnified morphology morphology of Zone of Zone III, respectively. I, (c) the magnified morphology of Zone II, and (d) the magnified morphology of Zone III, respectively. Metals 2017, 7, 500 4 of 9

Table 2. EDS (energy dispersive spectroscope) chemical analysis (at %) of different positions in Figure2. Metals 2017, 7, 500 4 of 9 Spots Y Al Si N O Possible Phases Table 2. EDS (energy dispersive spectroscope) chemical analysis (at %) of different positions in Figure 2. A 8.41 2.67 27.70 41.18 20.03 Y–Si–Al–O–N phase BSpots 1.80 Y 3.16Al 44.47Si N45.25 O 5.32 Possible Phasesβ-Sialon C 4.77 0.97 48.26 46.00 - β-Si N A 8.41 2.67 27.70 41.18 20.03 Y–Si–Al–O–N phase3 4 B 1.80 3.16 44.47 45.25 5.32 β-Sialon Figure3a,b show theC morphologies 4.77 0.97 of48.26 the P-Si46.003N 4/D-Si - 3N4 jointsβ-Si using3N4 Yb-SiAlON glass solder. No cracks resulting from thermal expansion mismatch appeared at the interfaces between the seam and ceramicFigure substrates. 3a,b show It the is also morphologies noted that of the the ribbon-like P-Si3N4/D-Si inﬁltration3N4 joints using zone Yb-SiAlON at about500 glassµ msolder. was also No cracks resulting from thermal expansion mismatch appeared at the interfaces between the seam formed on the porous Si3N4 ceramic side. Figure3c shows the magniﬁed morphology of the interface and ceramic substrates. It is also noted that the ribbon-like infiltration zone at about 500 μm was also between the seam and the dense Si3N4, where an interlocked microstructure can be observed at the formed on the porous Si3N4 ceramic side. Figure 3c shows the magnified morphology of the interface interface, which was mainly attributed to the concentration gradient between the intergranular phase between the seam and the dense Si3N4, where an interlocked microstructure can be observed at the in theinterface, matrix and which the glasswas mainly solder attributed composition. to the It hasconcentration been reported gradient that between the Yb–Si–Al–O the intergranular liquid is able to transformphase in intothe Yb–Si–Al–O–Nmatrix and the glass liquid solder system compos dueition. to the It dissolutionhas been reported of α-Si that3N 4theduring Yb–Si–Al–O the joining processliquid [24 ],is andable to the transform undissolved into Yb–Si–Al–O–Nα-Si3N4 may liquid be converted system due to toβ the-SiAlON dissolution above of α a-Si temperature3N4 during of ◦ 1500 theC via joining dissolution-precipitation process [24], and the or undissolved act as a heterogeneous α-Si3N4 may core.be converted Diffusion to of β the-SiAlON molten above glass afrom temperature of 1500 °C via dissolution-precipitation or act as a heterogeneous core. Diffusion of the the joining interface into the dense Si3N4 substrates was obvious, indicating that reaction occurs at the liquid/solidmolten glass interface.from the joining Composites interface of into glass/ the βdense-SiAlON-ceramic Si3N4 substrates formed was obviou in thes, indicating seam (Figure that 3d). reaction occurs at the liquid/solid interface. Composites of glass/β-SiAlON-ceramic formed in the Figure3e shows that most of molten glass penetrated into the porous Si 3N4 through its connected seam (Figure 3d). Figure 3e shows that most of molten glass penetrated into the porous Si3N4 pore. The porous nature of the porous Si3N4 was destroyed, while it contributed to the formation of a through its connected pore. The porous nature of the porous Si3N4 was destroyed, while it densecontributed joint interface. to the formation of a dense joint interface. Afterwards,Afterwards, the the XRD XRD patterns patterns of of the the joints joints usingusing two different different glass glass solders solders are areindicated indicated in in FigureFigure4. It is4. provedIt is proved that thatα -Siα-Si3N3N44 transferstransfers to to β-SiAlONβ-SiAlON during during the bonding the bonding process. process. In addition, In addition, the the crystallizationcrystallization of of Y Y2Si2Si3O33ON34 Nand4 and Yb2Si Yb3O23SiN34 Ophases3N4 phases were detected weredetected in these two in these different two joints, different joints,respectively. respectively.

Figure 3. Joint of P-Si3N4/D-Si3N4 using Yb-SiAlON glass solder at 1600 °C for◦ 30 min: (a,b) low and Figure 3. Joint of P-Si3N4/D-Si3N4 using Yb-SiAlON glass solder at 1600 C for 30 min: (a,b) low high magnification, (c) the magnified micrograph of the interface between D-Si3N4 and the seam and high magnification, (c) the magnified micrograph of the interface between D-Si N and the seam (Zone I), (d) the magnified micrograph of the seam (Zone II), and (e) the magnified micrograph3 4 of the (Zone I), (d) the magnified micrograph of the seam (Zone II), and (e) the magnified micrograph of the interface between P-Si3N4 and the seam (Zone III), respectively. interface between P-Si3N4 and the seam (Zone III), respectively. Metals 2017, 7, 500 5 of 9 Metals 2017, 7, 500 5 of 9

Metals 2017, 7, 500 5 of 9

Figure 4. XRD (X-ray diffraction) patterns taken from the filler area of the ceramics joints bonded by Figure 4. XRD (X-ray diffraction) patterns taken from the filler area of the ceramics joints bonded by (a) Y-SiAlON and (b) Yb-SiAlON, respectively. (a) Y-SiAlON and (b) Yb-SiAlON, respectively. Figure 4. XRD (X-ray diffraction) patterns taken from the filler area of the ceramics joints bonded by 3.2. Bending(a) Y-SiAlON Strength and of (b) the Yb-SiAlON, Joints respectively. 3.2. Bending Strength of the Joints 3.2.Figure Bending 5 Strengthshows the of the bending Joints strengths of the P-Si3N4/D-Si3N4 joints using RE–Si–Al–O–N (RE = Y orFigure Yb) glasses5 shows and the the bending typical fracture strengths morphology of the P-Si. 3WhenN4/D-Si the 3Y-SiAlONN4 joints usingglass was RE–Si–Al–O–N applied, the (roomRE = Y Figure 5 shows the bending strengths of the P-Si3N4/D-Si3N4 joints using RE–Si–Al–O–N (RE = Y or Yb)temperature glasses and bending the typical strength fracture of the joints morphology. were significantly When the improved Y-SiAlON with glass the was increase applied, of bonding the room or Yb) glasses and the typical fracture morphology. When the Y-SiAlON glass was applied, the room temperaturetemperature, bending from 56 strength MPa at of1550 the °C joints to 103 were MPa significantly at 1650 °C. The improved maximum with bending the increase strength of bonding of the temperature bending strength of the◦ joints were significantly◦ improved with the increase of bonding temperature,bonding joints from accounts 56 MPa for at 155073% ofC the to bending 103 MPa strength at 1650 ofC. the The porous maximum Si3N4 substrate. bending strengthTwo factors, of the temperature, from 56 MPa at 1550 °C to 103 MPa at 1650 °C. The maximum bending strength of the bondingincluding joints the accounts formation for of 73%the interlocking of the bending acicular strength β-SiAlON of the grain porous network Si3N and4 substrate. pore-free Two interface, factors, bonding joints accounts for 73% of the bending strength of the porous Si3N4 substrate. Two factors, includingwereincluding thought the the formation toformation be responsible of of the the interlocking interlocking for the high acicular strengths ββ-SiAlON-SiAlON of the grain joints grain network [21]. network When and and pore-freethe pore-freeYb-SiAlON interface, interface, glass werewaswere thought used, thought a to slight be to responsiblebe change responsible in for performance for the the high high strengths strengthswas found, of of the the and joints joints a maximum [ 21[21].]. WhenWhen strength the Yb-SiAlON of 88 MPa glass glass was was used,achievedwas a slight used, at change1600a slight °C, in changewhich performance wasin performance 62% wasof the found, bendingwas found, and strength a maximumand a ofmaximum the strengthporous strength Si of3N 884 substrate.of MPa 88 MPa was Fracture was achieved analysis◦ suggested that all the joints broke on the porous Si3N4 substrate side. Therefore, a typical at 1600achievedC, which at 1600 was °C, 62%which of was the 62% bending of thestrength bending strength of the porous of the porous Si3N4 substrate.Si3N4 substrate. Fracture Fracture analysis morphology of the joint fracture was selected and is shown 3in 4Figure 5b. Figure 5c displays the local suggestedanalysis that suggested all the jointsthat all broke the joints on the broke porous on the Si3 Nporous4 substrate Si N substrate side. Therefore, side. Therefore, a typical a morphologytypical of themagnifiedmorphology joint fracture region of the of was jointthe selected fracturefracture andwassurface, isselected shown and anda inmi isxed Figure shown fracture5 b.in Figure Figure path 5b.of5c glassFigure displays solder 5c displays the and local rodlike the magnified local Si 3N4 canmagnified be seen. region Thus, of the the fracture fracture locationsurface, andwas a inmi thexed infiltrationfracture path zone of glass of the solder porous and rodlikeSi3N4 substrate. Si3N4 region of the fracture surface, and a mixed fracture path of glass solder and rodlike Si3N4 can be seen. Thoughcan be theseen. infiltration Thus, the offracture the glass location solder was can in lead the toinfiltration the bonding zone of of the the porous porous Si Si3N3N4 4and substrate. the seam, Thus, the fracture location was in the infiltration zone of the porous Si3N4 substrate. Though the it Thoughcan also the break infiltration the skeletal of the structure glass solder of thecan porouslead to theSi3N bonding4 substrate, of the as porous mentioned Si3N4 above.and the Thus, seam, it is infiltration of the glass solder can lead to the bonding of the porous Si3N4 and the seam, it can also expectedit can also that break the thejoining skeletal can structure be improved of the porousby optimizing Si3N4 substrate, the infiltration as mentioned of the above. glass Thus, solders it is in a break the skeletal structure of the porous Si N substrate, as mentioned above. Thus, it is expected futureexpected work. that A thedetailed joining morphology can be improved of the 3 byjoints4 optimizing and corresponding the infiltration illustration of the glass is providedsolders in in a the that the joining can be improved by optimizing the infiltration of the glass solders in a future work. followingfuture work. section. A detailed morphology of the joints and corresponding illustration is provided in the A detailedfollowing morphology section. of the joints and corresponding illustration is provided in the following section.

Figure 5. The function of the bending strength of the P-Si3N3 4/D-Si4 3N34 joints4 as bonding temperatures FigureFigure 5. The5. The function function of of the the bending bending strength strength of of the the P-Si P-Si3NN4/D-Si/D-Si 3NN 4jointsjoints as as bonding bonding temperatures temperatures (a), the fracture surface of the joint bonded with Y-SiAlON at 1650 °C (b), and the magnified (a),( thea), the fracture fracture surface surface of the of joint the bondedjoint bonded with Y-SiAlON with Y-SiAlON at 1650 ◦atC( 1650b), and °C the(b), magniﬁed and the magnified micrograph micrograph of the fracture surface (c). of themicrograph fracture surfaceof the fracture (c). surface (c).

Metals 2017, 7, 500 6 of 9 Metals 2017, 7, 500 6 of 9

3.3. Effect of Bonding Temperatures onon MicrostructureMicrostructure EvolutionEvolution ofof thethe JointsJoints Figure6 displays the images of the P-Si N /D-Si N joints bonded by Y-SiAlON glass solder Figure 6 displays the images of the P-Si33N44/D-Si3N34 joints4 bonded by Y-SiAlON glass solder at µ atdifferent different temperatures temperatures for for30 30min. min. Although Although the thewidths widths of the of infiltration the infiltration zones zones were were 426 μ 426m, 428m, µ µ 428μm, andm, and436 436μm, m,which which changed changed slightly slightly as as the the bonding bonding temperature temperature increased, increased, many white ◦ ◦ ribbon-like “flowing “flowing channels” formed formed in in the infilt infiltrationration zone zone when when joined joined at at 1600 °CC and and 1650 1650 °C.C. The heterogeneous solder glass diffused and solidified in the porous Si N accounts for the formation The heterogeneous solder glass diffused and solidified in the porous3 4 Si3N4 accounts for the offormation the morphology. of the morphology. Therefore, Therefore, it can be deduced it can be that deduced the infiltration that the infiltration zone contains zone a contains white Y-SiAlON a white glass phase, which was embedded with fine-grain β-Sialon and β-Si N . Remnant pores in the Y-SiAlON glass phase, which was embedded with fine-grain β-Sialon and3 β4-Si3N4. Remnant pores in ◦ ◦ interfacethe interface of theof the infiltration infiltration zone/interlayer zone/interlayer (joining (joining at at 1550 1550 C°C and and 1600 1600 °C)C) cancan bebe observedobserved in ◦ Figure6 6d,e.d,e. However,However, whenwhen thethe temperaturetemperature waswas asas highhigh asas 16501650 °CC (Figure 66f),f), thethe whitewhite Y-SiAlONY-SiAlON glass diffused into the porous Si N substrate and filled the pores of the surface to form a continuous glass diffused into the porous Si33N4 substrate and filled the pores of the surface to form a continuous ◦ interface. ThisThis improvement improvement might might be attributedbe attributed to the to low the viscosity low viscosity of the glassof the at 1650glass C.at Therefore,1650 °C. theTherefore, bonding the strength bonding was strength effectively was enhanced.effectively enhanced.

Figure 6. The microstructure of P-Si3N4/Y-SiAlON/D-Si3N4 joints bonding at (a,d) 1550 °C, (b,e) 1600◦ Figure 6. The microstructure of P-Si3N4/Y-SiAlON/D-Si3N4 joints bonding at (a,d) 1550 C, (°C,b,e and) 1600 (c,◦f)C, 1650 and °C (c ,forf) 1650 30 min.◦C for 30 min.

A similar characterization of the joints using Yb-SiAlON glass solder at different temperatures A similar characterization of the joints using Yb-SiAlON glass solder at different temperatures is displayed in Figure 7. In addition, measurement of the thicknesses of the infiltration zones was is displayed in Figure7. In addition, measurement of the thicknesses of the infiltration zones was performed under SEM, and the thicknesses were determined to be 433 μm, 512 μm, and 427 μm at performed under SEM, and the thicknesses were determined to be 433 µm, 512 µm, and 427 µm at 1550 °C, 1600 °C, and 1650 °C, respectively. The distribution of white oxynitride glass in the 1550 ◦C, 1600 ◦C, and 1650 ◦C, respectively. The distribution of white oxynitride glass in the infiltration infiltration zones was inhomogeneous. The morphologies of the interface between the infiltration zones was inhomogeneous. The morphologies of the interface between the infiltration zone and the zone and the seam bonded at different temperatures are displayed from Figure 7d to Figure 7f. In the seam bonded at different temperatures are displayed from Figure7d to Figure7f. In the figures, figures, remnant pores can be observed at the interface. This may be attributed to the infiltration of remnant pores can be observed at the interface. This may be attributed to the infiltration of liquid liquid glass into the porous Si3N4 substrate. As a result, during cooling, the glass solder was not glassprovided into promptly the porous for Si filling3N4 substrate. the interface As a between result, during porous cooling, substrate the and glass seam, solder which was was not similar provided to promptlythe use of for Y-SiAlON filling the glass interface solder between (Figure porous 6d,e). substrateIf a continuous and seam, interface which was was formed similar between to the use the of Y-SiAlONporous substrate glass solder and (Figure seam 6(Figured,e). If a6f), continuous the bond interfaceing strength was formed would between improve. the porousTherefore, substrate it is andreasonable seam (Figure to assume6f), the the bonding lower bonding strength wouldstrength improve.s of these Therefore, joints. Figure it is reasonable 8 shows tothe assume magnified the lowermorphologies bonding of strengths the seam of thesezones joints. at different Figure 8temper showsatures. the magnified It is worth morphologies pointing out of the that seam the zonesgrain β atshapes different of β temperatures.-SiAlON (submicrometer It is worth pointing diameter) out thatapparently the grain grow shapes with of the-SiAlON increase (submicrometer in bonding diameter) apparently grow with the increase in bonding temperature. The microstructure of the temperature. The microstructure of the seam zone similar to that of the dense Si3N4 ceramic suggests seamthat the zone joining similar process to that followed of the densethe mechanism Si3N4 ceramic of the suggests ceramic thatsintering the joining process: process oxide melt followed formed, the mechanismsilicon nitride of dissolved the ceramic to sinteringform oxynitride process: glasses, oxide meltand the formed, β-phase silicon then nitride nucleated. dissolved Nevertheless, to form oxynitride glasses, and the β-phase then nucleated. Nevertheless, most of the glass liquid would most of the glass liquid would infiltrate into the porous Si3N4 substrate, and the redundant glass infiltrate into the porous Si3N4 substrate, and the redundant glass formed the intergranular phase in formed the intergranular phase in the joint, which is different from the process of sintering Si3N4 theceramics. joint, which is different from the process of sintering Si3N4 ceramics.

Metals 2017, 7, 500 7 of 9 Metals 2017, 7, 500 7 of 9 Metals 2017, 7, 500 7 of 9

Figure 7. The microstructure of the joints bonding at (a,d) 1550 °C, (b,e) 1600 °C, and (c,f) 1650 °C for Figure 7. The microstructure of the joints bonding at (a,d) 1550 ◦C, (b,e) 1600 ◦C, and (c,f) 1650 ◦C for Figure30 min 7.using The Yb-SiAlONmicrostructure glass of solder. the joints bonding at (a,d) 1550 °C, (b,e) 1600 °C, and (c,f) 1650 °C for 30 min using Yb-SiAlON glass solder. 30 min using Yb-SiAlON glass solder.

Figure 8. SEM micrographs of Yb-SiAlON glass solder in the seam zone at different bonding Figuretemperatures 8. SEMSEM (a ) micrographs1550 micrographs °C, (b) 1600of of Yb-SiAlON Yb-SiAlON°C, and (c) 1650glass glass °C. solder solder in in the the seam seam zone at different bonding temperatures (a) 1550 ◦°C,C, ( b) 1600 °C,◦C, and ( cc)) 1650 1650 °C.◦C. Above all, the microstructural evolution of the P-Si3N4/D-Si3N4 joints using RE–Si–Al–O–N (RE = Y orAbove Yb) oxynitride all, the microstructural glasses could beevolution described of asthe follows: P-Si3N4 /D-Sithe glass3N4 jointsRE2O 3using-Al2O 3RE–Si–Al–O–N-SiO2 system began (RE Above all, the microstructural evolution of the P-Si3N4/D-Si3N4 joints using RE–Si–Al–O–N to= Y melt or Yb) at aoxynitride eutectic temperature, glasses could which be described is approx asimately follows: 1350 the glass°C in RE this2O system3-Al2O3 -SiO[21],2 andsystem densified began (RE = Y or Yb) oxynitride glasses could be described as follows: the glass RE2O3-Al2O3-SiO2 theto melt joint at during a eutectic the joiningtemperature, process. which With is the approx tempimatelyerature rose,1350 °Cthe in glass this liqui systemd in [21], the seamand densified began to system began to melt at a eutectic temperature, which is approximately 1350 ◦C in this system [21], flowthe joint and during even infiltrate the joining into process. the porous With Si 3theN4 ceramic,temperature resulting rose, inthe the glass reduction liquid inof thethe seamglass beganliquid toin and densified the joint during the joining process. With the temperature rose, the glass liquid in flowthe seam. and evenWhen infiltrate the temperature into the porous further Si rose3N4 upceramic, to bonding resulting temperature in the reduction and as theof the temperature glass liquid was in the seam began to flow and even infiltrate into the porous Si3N4 ceramic, resulting in the reduction maintained,the seam. When the theα-Si temperature3N4 powders further dissolved rose upinto to bondingthe liquid temperature to form RE–Si–Al–O–N and as the temperature glasses. wasThe of the glass liquid in the seam. When the temperature further rose up to bonding temperature maintained,infiltration zone the formedα-Si3N4 onpowders the porous dissolved Si3N4 substrate into the sideliquid due to to form its porous RE–Si–Al–O–N microstructure glasses. and The the and as the temperature was maintained, the α-Si3N4 powders dissolved into the liquid to form flowabilityinfiltration zoneof the formed oxynitride on the glass porous solders. Si3 NAt4 substratedifferent joiningside due temperatures, to its porous the microstructure morphologies and of the RE–Si–Al–O–N glasses. The infiltration zone formed on the porous Si3N4 substrate side due to flowabilityinfiltration ofzone the wereoxynitride different, glass which solders. may At bediff closelyerent joining related temperatures, to the viscosity the ofmorphologies the liquid glasses of the its porous microstructure and the flowability of the oxynitride glass solders. At different joining andinfiltration the type zone of rare were earth different, elements. which This may has beyet closely to be further related studied to the viscosityin subsequent of the research liquid glasses work. temperatures, the morphologies of the infiltration zone were different, which may be closely related to andMeanwhile, the type on of therare dense earth Sielements.3N4 substrate This side,has yet it wasto be observed further studied that an ininterlocked subsequent microstructure research work. of the viscosity of the liquid glasses and the type of rare earth elements. This has yet to be further studied theMeanwhile, interface on was the produced dense Si3 Ndue4 substrate to the diffusion side, it was of liquid observed phase that driven an interlocked by concentration microstructure gradient. of in subsequent research work. Meanwhile, on the dense Si3N4 substrate side, it was observed that an Thethe interfacestrength wasof the produced joint would due toimprove the diffusion because of liquidof the phaseincreased driven local by densityconcentration of the gradient.interface interlocked microstructure of the interface was produced due to the diffusion of liquid phase driven by region,The strength and glass of the solders joint couldwould also improve be used because to bond of thethe porousincreased Si3N local4 ceramics density after of thesealing interface their concentration gradient. The strength of the joint would improve because of the increased local density region,surface pores.and glass solders could also be used to bond the porous Si3N4 ceramics after sealing their surface pores.

Metals 2017, 7, 500 8 of 9

of the interface region, and glass solders could also be used to bond the porous Si3N4 ceramics after sealing their surface pores.

4. Conclusions

In the present work, joining of Si3N4 ceramics has been successfully performed using RE–Si–Al–O–N (RE = Y or Yb) glass solders, at joining temperatures of 1550 ◦C, 1600 ◦C, and 1650 ◦C, each held for 30 min. The results lead to the following conclusions:

(1) RE–Si–Al–O–N (RE = Y or Yb) glass solders were successfully used to bond porous Si3N4 to dense Si3N4 ceramics at high temperature. The maximum bending strengths of the joints bonded using Y-SiAlON and Yb-SiAlON glass solders were measured to be 103 MPa and 88 MPa. The bending strength of the joints accounts for 73% and 62% of the bending strength of the porous Si3N4 substrate, respectively. (2) The microstructure of such joints consisted of a diffusion zone, an inﬁltration zone, and a seam zone. During the bonding process, an interlocked interface formed on the dense Si3N4 substrate side, and an inﬁltration zone arose on the porous Si3N4 substrate side. In the seam zone, α-Si3N4 partly transformed into β-SiAlON to form β-SiAlON-glass composite.

With the increase in bonding temperature, the microstructure evolution of the joint was dominated by the following factors: (i) The flowability of liquid oxyniride glass allowed for the rapid bonding of the ceramic substrates; (ii) The capillary action of the porous Si3N4 substrate contributed to the infiltration of glass solders. A small amount of infiltration is good for sealing porous Si3N4, while excessive infiltration may cause pores at the interface of the porous Si3N4 substrate side and weaken the bonding strength.

Acknowledgments: This work was financially supported by the National Natural Science Foundation of China (NSFC) under Grant No. 51621091 and U1537206. Author Contributions: The experiments were carried out by Ling Li, Liangbo Sun, and Xinhua Wang. Ling Li, Jie Zhang, and Chunfeng Liu conceived and designed the experiments. Ling Li, Liangbo Sun, and Xuanzhi Wang analyzed the data. The manuscript was written by Ling Li and revised by Liangbo Sun, Jie Zhang, and Chunfeng Liu. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Riley, F.L. Silicon nitride and related materials. J. Am. Ceram. Soc. 2000, 83, 245–265. [CrossRef] 2. Liu, X.J.; Huang, Z.Y.; Ge, Q.M.; Sun, X.W.; Huang, L.P. Microstructure and mechanical properties of silicon

nitride ceramics prepared by pressureless sintering with MgO-Al2O3-SiO2 as sintering additive. J. Eur. Ceram. Soc. 2005, 25, 3353–3359. [CrossRef] 3. Krstic, Z.; Krstic, V.D. Silicon nitride: The engineering material of the future. J. Mater. Sci. 2011, 47, 535–552. [CrossRef] 4. Hou, X.; Wang, E.; Li, B.; Chen, J.; Chou, K.-C. Corrosion behavior of porous silicon nitride ceramics in different atmospheres. Ceram. Int. 2017, 43, 4344–4352. [CrossRef]

5. Kawai, C.; Yamakawa, A. Effect of porosity and microstructure on the strength of Si3N4: Designed microstructure for high strength, high thermal shock resistance, and facile machining. J. Am. Ceram. Soc. 1997, 80, 2705–2708. [CrossRef]

6. Wang, H.; Yu, J.; Zhang, J.; Zhang, D. Preparation and properties of pressureless-sintered porous Si3N4. J. Mater. Sci. 2010, 45, 3671–3676. [CrossRef]

7. Wang, C.; Wang, H.; Qiao, R.; Zhang, C.; Chen, L. Fabrication and thermal shock resistance of β-Si3N4-based environmental barrier coating on porous Si3N4 ceramic. Ceram. Int. 2016, 42, 14222–14227. [CrossRef] 8. Wang, C.; Fan, L.; Fan, J.; Zhang, D.; Wang, H. Effect of spraying power on microstructure and properties of

supersonic plasma sprayed Al2O3 coating on porous Si3N4 substrate. J. Alloys Compd. 2013, 559, 152–157. [CrossRef] Metals 2017, 7, 500 9 of 9

9. Zhuang, Y.L.; Lin, T.S.; Wang, S.J.; He, P.; Sekulic, D.P.; Jia, D.C. Microstructural and mechanical properties of

porous Si3N4 carbon coated ceramic brazed with a cobalt-silicon ﬁller. J. Eur. Ceram. Soc. 2017, 37, 3293–3301. [CrossRef]

10. He, Y.; Zhang, J.; Li, X. Characterization of the Si3N4/Si3N4 joints fabricated using particles modiﬁed braze. Mater. Sci. Eng. A 2014, 616, 107–115. [CrossRef]

11. Wang, T.; Ivas, T.; Lee, W.; Leinenbach, C.; Zhang, J. Relief of the residual stresses in Si3N4/Invar joints by multi-layered braze structure-experiments and simulation. Ceram. Int. 2016, 42, 7080–7087. [CrossRef] 12. Li, J.; Liu, L.; Wu, Y.; Li, Z.; Zhang, W.; Hu, W. Microstructure of high temperature Ti-based brazing alloys and wettability on SiC ceramic. Mater. Des. 2009, 30, 275–279. [CrossRef]

13. Lemusruiz, J.; Leonpatino, C.; Aguilarreyes, E. Interface behaviour during the self-joining of Si3N4 using a Nb-foil interlayer. Scr. Mater. 2006, 54, 1339–1343. [CrossRef]

14. Xie, R.; Huang, L.; Fu, X. Bond strength and microstructural investigation on Si3N4/Si3N4 joint bonded with glass-ceramic. J. Mater. Sci. Lett. 1998, 17, 761–763. [CrossRef] 15. Xie, R.; Huang, L.; Fu, X.; Chen, Y. Effects of adhesive composition on bond strength of joined silicon nitride ceramics. J. Eur. Ceram. Soc. 1998, 18, 901–905. [CrossRef] 16. Xie, R.; Huang, L.; Chen, Y.; Fu, X. Effects of chemical compositions of adhesive and joining processes on

bond strength of Si3N4/Si3N4 joints. Ceram. Int. 1999, 25, 101–105. [CrossRef] 17. Gopal, M.; Sixta, M.; De Jonghe, L.; Thomas, G. Seamless joining of silicon nitride ceramics. J. Am. Ceram. Soc. 2001, 84, 708–712. [CrossRef] 18. Gopal, M.; De Jonghe, L.C.; Thomas, G. Silicon nitride joining using rare-earth reaction sintering. Scr. Mater. 1997, 36, 455–460. [CrossRef]

19. Iwamoto, N.; Umesaki, N.; Haibara, Y. Silicon nitride joining with glass solder in the system CaO-SiO2-TiO2. Int. J. High Technol. Ceram. 1987, 3, 176. [CrossRef] 20. Johnson, S.M.; Rowcliffe, D.J. Mechanical properties of joined silicon nitride. J. Am. Ceram. Soc. 1985, 68, 468–472. [CrossRef] 21. Walls, P.A.; Ueki, M. Mechanical properties of β-sialon ceramics joined using composite β-sialon-glass adhesives. J. Am. Ceram. Soc. 1995, 78, 999–1005. [CrossRef] 22. Walls, P.A.; Ueki, M. Joining sialon ceramics using composite β-sialon-glass adhesives. J. Am. Ceram. Soc. 1992, 75, 2491–2497. [CrossRef] 23. Weldon, L.M.; Hampshire, S.; Pomeroy, M.J. Joining of ceramics using oxide and oxynitride glasses in the Y-Sialon system. J. Eur. Ceram. Soc. 1997, 17, 1941–1947. [CrossRef] 24. Saito, N.; Kai, K.; Furusho, S.; Nakashima, K.; Mori, K.; Shimizu, F. Properties of nitrogen-containing yttria-Alumina-Silica melts and glasses. J. Am. Ceram. Soc. 2003, 86, 711–716. [CrossRef]

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).