J.Am. Ceran.So,89图1615-l620(2006) DOl:10.ll1551-2916.200600911.x o 2006 The American Ceramic Society ourn Role of Rare-Earth Oxide Additives on Mechanical Properties and Oxidation Behavior of Si3 N,/BN Fibrous Monolith Ceramics SigrunN. Karlsdottir'and John W.Halloran Materials Science and Engineering Department, University of Michigan. Ann Arbor, Michigan 48109-2136 rare-earth oxides ytterbium oxide(Yb2 O3)and lanthanum crystallize the grain-boundary phase. -Liu and Nemat-Nas- (La2O3)were used as additives in fibrous monolithic(FM) ser studied the microstructure of an in Si3N4BN tes to study their individual effect on flexural nitride. sintered with the rare-earth oxides lanthanum trength and oxidation behavior of the composite. Two LayO3) and Y,O3. They found crystalline grain-bot tives for the Si3,a and bn being either 8 wt%oYb,O3 or 8 wt% ases LasSiO12N and YssiO12N, formed at grain boundaries of the si, N,. Cinibul La,O3 Four-point flexural testing and static oxidation exper achieved good oxidation resistance and mechanical properties iments at 1400C in dry air for 10 h were performed. The ma- of monolithic Si3 N4 at high temperatures by sintering the sin terial with Yb2O3 showed a high flexural strength, graceful with various rare-earth oxides and silicon dioxide additives and failure, and comparable strength to reported Si3N4/BN FMs then heat treated it to form crystalline rare-earth silicate phase with 6 wt%Y2O3 and 2 wt% AlO3. The material with LazO Park et al.used Yb2O3 as a sintering aid to enhance the me- showed lower flexural strength and brittle failure in the majority chanical properties of Si3N4. They found that the amount of of the samples; this was believed to be related to the hydration of Yb O3 had considerable effects on the microstructural evolution LazO3 from the rare-earth apatite phase, LasSi3O12N, resulting and the composition of the secondary phase in the grain bound- in lanthanum hydrate crystals on the side surfaces of the sample ary. Different crystalline grain boundary phases were formed for nd disintegration of the material. The surface of the fmla amounts of Yb2O3, for 8 wt% Yb2O3 crystalline sample after oxidation showed severe oxidation. In contrast, the was formed at the grain boundary along with a glassy oxidation test of the FMs with Yb2O3 revealed a thin oxide scale he size of the Si3N4 grains also varied with the amoun Sio ng small Yb Si,0, on the SiN, cells but large Yb2 of Yb,O3. These changes influenced the mechanical properties of the material at room temperature and elevated temperature, showed 100 um recession in the bn cell boundary and an e the flexural strength increased with the increased amount of 4 um oxide scale on Si3N4 cells. Yb O3 used Other researchers have used sintering additives in Si3N4 to ncrease oxidation resistance. Lee and Readey increased the oxidation resistance of Si3 Na by using Y b2O3 as an additive and . Introduction then generating a protective ytterbium silicate (Yb2 Si2O7) skin F IBROUS MONOLITHIC(FM) ceramics are laminates with a 3D by a controlled oxidation process associated with the reaction structure. Because of their unique structure, they fail in between the Si3N4 oxidation products SiO2 and Yb2O3. Yb2 non-catastrophic way and thus have been considered as prom- Si2O, has also been reported as one of the main oxidation prod- ising materials for structural applications. FM ceramics have ucts formed on the surface of nanocomposite Si3NA-SiC with a fibrous texture and consist of a strong cell surrounded by a Yb2O3 as a sintering additive. weaker boundary phase. The most thoroughly investigated FM Just as rare-earth oxides have been shown to confer suitable ceramic system is the Si3N4/BN FMs. This system is con- grain boundary properties to Si N4 and enhance mechanical properties and oxidation resistance, rare-earth oxides can be ex strength at elevated temperatures and thermal shock resist- pected to confer suitable grain boundary properties to the si3n4 ance.6 The additives yttrium oxide(Y,0.)and aluminum ox- cell for the Si,Na/BN FMs and also possibly increase the sta- de(Al O)are conventionally used in the cells of the SigN4 in bility of the BN cell boundary phase at elevated temperatures the si3 N4/BN FMs as sintering aids It is well known that during However, so far, no research has been carried out on this aspect sintering of monolithic Si,N4(containing sintering additives), a In this paper, we investigated the mechanical properties and liquid phase forms when the sintering additives react with SiO2 oxidation behavior of SinA/BN FMs with the rare-earth oxide that coats the Si3N4 particles. After sintering, this liquid phase is additives, Yb203 and Laz O3, by flexural strength testing at room usually retained in a glassy intergranu lar phase. 9, I For SisN BN FMs with Y2O3 and Al2O3 as additives, the glassy phas 10h. forms and is known to migrate into the bn cell boundaries during hot pressing. This intergranular glassy phase influences the mechanical properties of the Si3N4/BN FMs at elevated II. Experimental Procedure temperatures. (1) Material Fabrication effort to enhance the mechanical properties of mono- Na at room temperature and at elevated temperatures. FM Si N/BN samples were fabricated firstly b by using coextru- esearchers have added additives to Si3N4 to in- sion to prepare green filaments. The filaments were then stacked le refractoriness of the tergranular phase and or to form a green billet. After a binder burnout step, the billets were hot pressed at 25 MPa for I h in a flowing N2 atmosphere at 1800oC. Detailed descriptions of the fabrication of FMs have M. Cinibulk--contributing editor been further described elsewhere Billets with two different compositions were prepared: 20 vol% BN/80 vol% Si3N4 with additives for the Si3 N4 and bn being Manuscript No 20810. Received July 27. 2005: d November 29. 2005. either 8 wt%o-Yb2O3(REacton, Alfa Aesar, Ward Hill, MA)or Author to wt%La2O3. Lanthanum hydrate, La(OH)3(Alfa Aesar). 1615
Role of Rare-Earth Oxide Additives on Mechanical Properties and Oxidation Behavior of Si3N4/BN Fibrous Monolith Ceramics Sigrun N. Karlsdottirw and John W. Halloran Materials Science and Engineering Department, University of Michigan, Ann Arbor, Michigan 48109-2136 The rare-earth oxides ytterbium oxide (Yb2O3) and lanthanum oxide (La2O3) were used as additives in fibrous monolithic (FM) Si3N4/BN composites to study their individual effect on flexural strength and oxidation behavior of the composite. Two compositions were prepared: 20 vol% BN/80 vol% Si3N4 with additives for the Si3N4 and BN being either 8 wt%Yb2O3 or 8 wt% La2O3. Four-point flexural testing and static oxidation experiments at 14001C in dry air for 10 h were performed. The material with Yb2O3 showed a high flexural strength, graceful failure, and comparable strength to reported Si3N4/BN FMs with 6 wt% Y2O3 and 2 wt% Al2O3. The material with La2O3 showed lower flexural strength and brittle failure in the majority of the samples; this was believed to be related to the hydration of La2O3 from the rare-earth apatite phase, La5Si3O12N, resulting in lanthanum hydrate crystals on the side surfaces of the samples and disintegration of the material. The surface of the FMLA sample after oxidation showed severe oxidation. In contrast, the oxidation test of the FMs with Yb2O3 revealed a thin oxide scale containing small Yb2Si2O7 on the Si3N4 cells but large Yb2- Si2O7 on the BN cell boundary. Also, microscopic analysis showed B100 lm recession in the BN cell boundary and an B4 lm oxide scale on Si3N4 cells. I. Introduction FIBROUS MONOLITHIC (FM) ceramics are laminates with a 3D structure. Because of their unique structure, they fail in a non-catastrophic way and thus have been considered as promising materials for structural applications.1–5 FM ceramics have a fibrous texture and consist of a strong cell surrounded by a weaker boundary phase. The most thoroughly investigated FM ceramic system is the Si3N4/BN FMs.1–8 This system is considered one of the most promising FMs because of its high strength at elevated temperatures and thermal shock resistance.3–6 The additives yttrium oxide (Y2O3) and aluminum oxide (Al2O3) are conventionally used in the cells of the Si3N4 in the Si3N4/BN FMs as sintering aids. It is well known that during sintering of monolithic Si3N4 (containing sintering additives), a liquid phase forms when the sintering additives react with SiO2 that coats the Si3N4 particles. After sintering, this liquid phase is usually retained in a glassy intergranular phase.9,10 For Si3N4/ BN FMs with Y2O3 and Al2O3 as additives, the glassy phase forms and is known to migrate into the BN cell boundaries during hot pressing.2,4 This intergranular glassy phase influences the mechanical properties of the Si3N4/BN FMs at elevated temperatures.3 In an effort to enhance the mechanical properties of monolithic Si3N4 at room temperature and at elevated temperatures, many researchers have added sintering additives to Si3N4 to increase the refractoriness of the glassy intergranular phase and/or crystallize the grain–boundary phase.11–15 Liu and Nemat-Nasser12 studied the microstructure of an in situ reinforced silicon nitride, sintered with the rare-earth oxides lanthanum oxide (La2O3) and Y2O3. They found crystalline grain–boundary phases La5Si3O12N and Y5Si3O12N, formed at grain pockets and two grain boundaries of the Si3N4. Cinibulk et al. 13,14 achieved good oxidation resistance and mechanical properties of monolithic Si3N4 at high temperatures by sintering the Si3N4 with various rare-earth oxides and silicon dioxide additives and then heat treated it to form crystalline rare-earth silicate phases. Park et al. 15 used Yb2O3 as a sintering aid to enhance the mechanical properties of Si3N4. They found that the amount of Yb2O3 had considerable effects on the microstructural evolution and the composition of the secondary phase in the grain boundary. Different crystalline grain boundary phases were formed for different amounts of Yb2O3; for 8 wt% Yb2O3 crystalline Yb2Si2O7 was formed at the grain boundary along with a glassy phase. The size of the Si3N4 grains also varied with the amount of Yb2O3. These changes influenced the mechanical properties of the material at room temperature and elevated temperature, i.e. the flexural strength increased with the increased amount of Yb2O3 used. Other researchers have used sintering additives in Si3N4 to increase oxidation resistance. Lee and Readey16 increased the oxidation resistance of Si3N4 by using Yb2O3 as an additive and then generating a protective ytterbium silicate (Yb2Si2O7) skin by a controlled oxidation process associated with the reaction between the Si3N4 oxidation products SiO2 and Yb2O3. Yb2- Si2O7 has also been reported as one of the main oxidation products formed on the surface of nanocomposite Si3N4–SiC with Yb2O3 as a sintering additive.17 Just as rare-earth oxides have been shown to confer suitable grain boundary properties to Si3N4 and enhance mechanical properties and oxidation resistance, rare-earth oxides can be expected to confer suitable grain boundary properties to the Si3N4 cell for the Si3N4/BN FMs and also possibly increase the stability of the BN cell boundary phase at elevated temperatures. However, so far, no research has been carried out on this aspect. In this paper, we investigated the mechanical properties and oxidation behavior of Si3N4/BN FMs with the rare-earth oxide additives, Yb2O3 and La2O3, by flexural strength testing at room temperature and static oxidation testing at 14001C in dry air for 10 h. II. Experimental Procedure (1) Material Fabrication FM Si3N4/BN samples were fabricated firstly by using coextrusion to prepare green filaments. The filaments were then stacked to form a green billet. After a binder burnout step, the billets were hot pressed at 25 MPa for 1 h in a flowing N2 atmosphere at 18001C. Detailed descriptions of the fabrication of FMs have been further described elsewhere.1 Billets with two different compositions were prepared: 20 vol% BN/80 vol% Si3N4 with additives for the Si3N4 and BN being either 8 wt%- Yb2O3 (REacton, Alfa Aesar, Ward Hill, MA) or 8 wt% La2O3. Lanthanum hydrate, La(OH)3 (Alfa Aesar), was 1615 Journal J. Am. Ceram. Soc., 89 [5] 1615–1620 (2006) DOI: 10.1111/j.1551-2916.2006.00911.x r 2006 The American Ceramic Society M. Cinibulk—contributing editor w Author to whom correspondence should be addressed. e-mail: nanna@umich.edu Manuscript No. 20810. Received July 27, 2005; approved November 29, 2005
16l6 Journal of the American Ceramic Society- dotter and halloran Vol. 89. No. 5 Table L. Composition and Densities of the fabricated Si3N4/ Table ll. Flexural Strength and WOF of Si3 N,/BN FMs with BN FMS Ytterbium oxide(FMYb)and Si3,/BN FMs with Lanthanum oxide(FmLa) Ytterbium oxide(wt%) oxide(wt%) Work-of-fracture (J/m") FMYB 8 3.09+0.1 FMYB FMLA 295+0.1 Average 2126 Standard FMLA used in place of the reactive La2 O3. Heating La(oH) in dry air Average Standard 61 our fabrication, the La(oh)3 then dehydrated during hot press- ing and transformed into LazO. both billets consisted of fila- ments stacked up, forming a 3D structure with x250 um Si3N4 ells(MIl. H.C. Starck, MA)uniaxially aligned and separated the FMla and FMyB The Si3 N4/BN FMs with by x15 um BN cell boundaries(6003, Advanced Ceramics Yb2O3 showed average hig ength and graceful fail orp, OH). The densities of the specimens were measured ure and comparable strengt ommercially available uniax using the Archimedes method, and the theoretical density of N FMs with 6 wt%Y,O3 and 2 wt% Al,O3, with the specimen was estimated by the rule of mixture where the the average flexural strength reported as 510+87 MPa. On the vol% of the bn phase was estimated to be 20 and the Si3 N4 other hand, the Si3 N4/BN FMs with La,O3 showed lower flex phase to be 80. The densities and compositions of the billets are ural strength and brittle failure in the majority of the samples. The FMYB had 97.8% theoretical density and FMLa had 94.6% theoretical density; this is not believed to be a large (2) Flexural Testing enough difference to explain completely the poor mechanical Flexural bars were prepared for four-point flexural testing. The Behavior of the FMLA samples. Figure I shows an example ofa billets were first ground with a 220 grit--diamond whe flexural response, a graceful failure, of the FMYB sample in a then cut into 2.2 mm x4.2 mmx 49 mm bars. The sides of the stress was 448 MPa and there was a large load drop where the sides of the bars were polished down to I um using a medium J/m2. Figure 2 shows an SEM micrograph of the side view of the room temperature in laboratory air using a computer-control corresponding FMYB sample, where the tensile initiation of the led, screw-driven, testing machine(Model 4483, Instron Corp, fracture can be seen along with crack defection and subsequent Canton, MA). The specimens were tested using a four-poin delamination cracking and sliding along the side surface. The flexural testing fixture with an inner and an outer span of 20 ane two fracture modes tensile and shear initiation were observed 40 mm and at a cross-head speed of O 5 mm/min. Load versus example of stress versus cross-head displacement curve of an eported here. Flexural strength is defined as the apparent flex FMLA sample that fractured by shear initiation and failed ural stress at load drop. Energy absorption capability gracefully. The apparent peak stress was 233 MPa for this sam- acterized by the woF, calculated by determining the are ple and the load drop was less compared with the tensile-initi ated fracture shown in Fig. 1. The retained apparent stress was 160 MPa and the WOF was 1304 J/m. Figure 4 shows the ross-sectional area of the sample. Scanning electron 1 d failure initiated in the flexural bar between the outer and inner amination cracking and sliding on the side surfaces of the tested flexural bars higher retained apparent stress than the tensile- initiated fracture samples. It was observed that all the samples that failed grace- (3) Oxidation Testing fully(non-catastrophically) required extensive crack interaction 1400c in dry ai for eo hn The fur nace was heated at a heati ng The average woF for FMLa was lower than for the FMri rate of 100C/min and then maintained at 1400.C for 10 h. Be- oxidation of the samples,2.2mm×4.2mm×20 manner and therefore there was were polished down to I um using a medium polishing diamond fewer delamination and A brick deflection, resulting in less energy disk. The specimens were then ultrasonically cleaned in acetone and dried before oxidation. The materials were characterized by X-ray diffractometry(XRD)before and after the oxidation test. position and morphology of an oxide layer produced after oxidation were characterized by sEM and X-ray energy 400 dispersive spectroscopy (XEDS) II. Results and discussion (1) Mechanical Properties FMs are laminates and can fail in two different modes durin flexural testing by shear initiation, where the shear stress be- tween the inner and outer loading pins in the middle of the beam exceeds the shear strength of the material, and by tensile initi- ation, where the tensile stress in the outer layer of the tensile 0.000080.170250.330420.500.580.67 urface exceeds the tensile strength of the material. Here we re- Crosshead Displacement [mm] port fracture of the FMs by two modes: tensile initiation and Fig 1. Flexural response of Si3N4/BN FMs with ytterbium oxide shear initiation. Table il gives the average strength and woF of(FMYB
used in place of the reactive La2O3. Heating La(OH)3 in dry air led to a progressive dehydration of the La(OH)3 to La2O3. 18 In our fabrication, the La(OH)3 then dehydrated during hot pressing and transformed into La2O3. Both billets consisted of filaments stacked up, forming a 3D structure with B250 mm Si3N4 cells (M11, H.C. Starck, MA) uniaxially aligned and separated by B15 mm BN cell boundaries (6003, Advanced Ceramics Corp., OH). The densities of the specimens were measured using the Archimedes method, and the theoretical density of the specimen was estimated by the rule of mixture where the vol% of the BN phase was estimated to be 20 and the Si3N4 phase to be 80. The densities and compositions of the billets are shown in Table I. (2) Flexural Testing Flexural bars were prepared for four-point flexural testing. The billets were first ground with a 220 grit—diamond wheel and then cut into 2.2 mm 4.2 mm 49 mm bars. The sides of the bars were chamfered to minimize machining flaws. The tensile sides of the bars were polished down to 1 mm using a medium polishing diamond disk. The flexural strength was measured at room temperature in laboratory air using a computer-controlled, screw-driven, testing machine (Model 4483, Instron Corp., Canton, MA). The specimens were tested using a four-point flexural testing fixture with an inner and an outer span of 20 and 40 mm and at a cross-head speed of 0.5 mm/min. Load versus cross-head deflection response and work of fracture (WOF) are reported here. Flexural strength is defined as the apparent flexural stress at load drop. Energy absorption capability is characterized by the WOF, calculated by determining the area under the load–cross-head deflection curve and dividing it by twice the cross-sectional area of the sample. Scanning electron microscopy (SEM) was used for examining crack deflection, and delamination cracking and sliding on the side surfaces of the tested flexural bars. (3) Oxidation Testing Oxidation studies were conducted in a vertical tube furnace at 14001C in dry air for 10 h. The furnace was heated at a heating rate of 1001C/min and then maintained at 14001C for 10 h. Before oxidation of the samples, 2.2 mm 4.2 mm 20 mm bars were polished down to 1 mm using a medium polishing diamond disk. The specimens were then ultrasonically cleaned in acetone and dried before oxidation. The materials were characterized by X-ray diffractometry (XRD) before and after the oxidation test. The composition and morphology of an oxide layer produced after oxidation were characterized by SEM and X-ray energydispersive spectroscopy (XEDS). III. Results and Discussion (1) Mechanical Properties FMs are laminates and can fail in two different modes during flexural testing by shear initiation, where the shear stress between the inner and outer loading pins in the middle of the beam exceeds the shear strength of the material, and by tensile initiation, where the tensile stress in the outer layer of the tensile surface exceeds the tensile strength of the material. Here, we report fracture of the FMs by two modes: tensile initiation and shear initiation. Table II gives the average strength and WOF of the FMLA and FMYB samples. The Si3N4/BN FMs with Yb2O3 showed average high flexural strength and graceful failure and comparable strength of commercially available uniaxially Si3N4/BN FMs with 6 wt% Y2O3 and 2 wt% Al2O3, with the average flexural strength reported as 510787 MPa.4 On the other hand, the Si3N4/BN FMs with La2O3 showed lower flexural strength and brittle failure in the majority of the samples. The FMYB had 97.8% theoretical density and FMLA had 94.6% theoretical density; this is not believed to be a large enough difference to explain completely the poor mechanical behavior of the FMLA samples. Figure 1 shows an example of a flexural response, a graceful failure, of the FMYB sample in a stress versus cross-head displacement curve. The apparent peak stress was 448 MPa and there was a large load drop where the retained apparent stress was B50 MPa and the WOF was 3037 J/m2 . Figure 2 shows an SEM micrograph of the side view of the corresponding FMYB sample, where the tensile initiation of the fracture can be seen along with crack deflection and subsequent delamination cracking and sliding along the side surface. The two fracture modes, tensile and shear initiation, were observed for both the FMYB and FMLA samples. Figure 3 shows an example of stress versus cross-head displacement curve of an FMLA sample that fractured by shear initiation and failed gracefully. The apparent peak stress was 233 MPa for this sample and the load drop was less compared with the tensile-initiated fracture shown in Fig. 1. The retained apparent stress was B160 MPa and the WOF was 1304 J/m2 . Figure 4 shows the side view of the corresponding FMLA sample where the shear failure initiated in the flexural bar between the outer and inner loading pins. The entire shear-initiated fractured samples had higher retained apparent stress than the tensile-initiated fracture samples. It was observed that all the samples that failed gracefully (non-catastrophically) required extensive crack interaction such as crack deflection, delamination cracking, and sliding. The average WOF for FMLA was lower than for the FMYB (Table II) because of the fact that the majority of the FMLA samples fractured in a brittle manner and therefore there was fewer delamination and crack deflection, resulting in less energy 0 100 200 300 400 500 0.00 0.08 0.17 0.25 0.33 0.42 0.50 0.58 0.67 Stress [MPa] Crosshead Displacement [mm] Fig. 1. Flexural response of Si3N4/BN FMs with ytterbium oxide (FMYB). Table I. Composition and Densities of the fabricated Si3N4/ BN FMs Ytterbium oxide (wt%) Lanthanum oxide (wt%) Measured r (g/cm3 ) Theoretical r (g/cm3 ) FMYB 8 — 3.0970.1 3.16 FMLA — 8 2.9570.1 3.12 Table II. Flexural Strength and WOF of Si3N4/BN FMs with Ytterbium oxide (FMYB) and Si3N4/BN FMs with Lanthanum oxide (FMLA) Stress (MPa) Work-of-fracture (J/m2 ) FMYB Average 340 2126 Standard 91 940 FMLA Average 298 1287 Standard 61 384 1616 Journal of the American Ceramic Society—Karlsdottir and Halloran Vol. 89, No. 5
May 2006 Rare-Earth Oxide Additives on Mechanical Properties and Oxidation Behavior dissipation. During an SEM/XEDS procedure, lanthanum- rich Side surface crystals were observed spread over the side surfaces of the FMLA specimens; see Fig. 5. XEDS analyses of these crystals nowed peaks of N, O, Si, and La. XRD analyses were also performed, where B-Si3 N4, BN, and small rare-earth apatite Lassi3o12N phases were identified. The rare-earth apatite phase. Las Si3O12N, was identified in grain-boundary phases in an in situ reinforced silicon nitride with La,o Tensile initiation he lanthanum-rich crystals growing from the side surfaces was 如mmH believed to be La(OH)3 crystals that were formed in air during hydration of La2O3 from the rare-earth apatite phase, Las. Fig.2. Scanning electron micrograph of a side view of an FMYB D3012N. This could not be confirmed with XRD analysis be- cause of the small amount of the crystals on the samples. Meadowcroft found that a strontium-doped lanthanum chro- mite prepared to comprise LacrO3+x mol% Sro disintegrated at room temperature, owing to the formation of La(oh)3, whicl was established by XRD analysis. The strontium was believed to have displaced the lanthanum in order to dope the lanthanum chromite, and then the free LayO, hydrated with time, where the 00 associated expansion caused the ceramic to disintegrate. The brittle behavior and low strength of the FMLa samples com- pared with the FMYB samples also gave us reason to believe that the ceramic disintegrated because of hydration of LayO3 available from the rare-earth apatite phase, LasSi3O12N (2) Characterization of the Oxidation Surface Layer The surface of the FMLa sample after oxidation for 10 h at 1400.C in dry air showed severe oxidation; SEM analysis of the 0.000080.170.250.330.420.500.58 Crosshead Displacement [mm] ered with burst bubbles and cracks. Because of the disintegra tion of the FMLA samples, we ot discuss the oxidation Fig 3. Flexural response of Si3N4/BN FMs with lanthanum oxide the FMla here in this paper but will focus on the oxidation the fmrb The oxidation behavior of SiNa ceramics with additive tems has been reported to be strongly dependent on the chemical composition of the oxidation surface layer formed by the oxidation products of the Si3 N4 and the sintering additives. 20 shows an Sem picture of th Side surface dized surface of FMYB after 10 h at 1400C in dry air where an oxide scale has formed. With XEDS and X-ray diffractiomet he oxidation product was identified to be mainly ytterbium sil icates (Yb2Si207). Large ytterbium silicates, 20 um, were found on the bn cell boundary but small ytterbium silicates were found on the Si3 N4 cells of 0.8 um. This can be seen in Figs. 6(b)and (c), which shows a higher magnification of the Shear initiation ytterbium silicates on the bn boundary phase and the sigN 加出知 cells in Fig. 6(a); the ytterbium silicates are the white flattened crystals in Figs. 6(aHc). XRD was performed before and after Fig 4. Scanning electron micrograph of a side view of an FMLA flex- the oxidation testing for FMYB. The XRD analysis of the un- iral bar showing a shear-initiated fracture between the outer and inner oxidized FMyB sample revealed B-Si3N4, BN, and Yb Si2O,N peaks, and B-si3N4, BN, Yb4si2O7N2, and Yb, peaks of SinA B SiaNA SE 102 OR Fig. 5. (a) Lanthanum-rich crystals formed on the side surface of FMLA samples. (b) Magnification of the box in(a) of the crystals formed
dissipation. During an SEM/XEDS procedure, lanthanum-rich crystals were observed spread over the side surfaces of the FMLA specimens; see Fig. 5. XEDS analyses of these crystals showed peaks of N, O, Si, and La. XRD analyses were also performed, where b-Si3N4, BN, and small rare-earth apatite La5Si3O12N phases were identified. The rare-earth apatite phase, La5Si3O12N, was identified in grain–boundary phases in an in situ reinforced silicon nitride with La2O3. 12 The composition of the lanthanum-rich crystals growing from the side surfaces was believed to be La(OH)3 crystals that were formed in air during hydration of La2O3 from the rare-earth apatite phase, La5- Si3O12N. This could not be confirmed with XRD analysis because of the small amount of the crystals on the samples. Meadowcroft19 found that a strontium-doped lanthanum chromite prepared to comprise LaCrO31x mol% SrO disintegrated at room temperature, owing to the formation of La(OH)3, which was established by XRD analysis. The strontium was believed to have displaced the lanthanum in order to dope the lanthanum chromite, and then the free La2O3 hydrated with time, where the associated expansion caused the ceramic to disintegrate. The brittle behavior and low strength of the FMLA samples compared with the FMYB samples also gave us reason to believe that the ceramic disintegrated because of hydration of La2O3, available from the rare-earth apatite phase, La5Si3O12N. (2) Characterization of the Oxidation Surface Layer The surface of the FMLA sample after oxidation for 10 h at 14001C in dry air showed severe oxidation; SEM analysis of the surface of the sample showed that the oxidation layer was covered with burst bubbles and cracks. Because of the disintegration of the FMLA samples, we will not discuss the oxidation of the FMLA here in this paper but will focus on the oxidation of the FMYB. The oxidation behavior of Si3N4 ceramics with additive systems has been reported to be strongly dependent on the chemical composition of the oxidation surface layer formed by the oxidation products of the Si3N4 and the sintering additives.20 Figure 6(a) shows an SEM picture of the top view of the oxidized surface of FMYB after 10 h at 14001C in dry air where an oxide scale has formed. With XEDS and X-ray diffractiometry, the oxidation product was identified to be mainly ytterbium silicates (Yb2Si2O7). Large ytterbium silicates, B20 mm, were found on the BN cell boundary but small ytterbium silicates were found on the Si3N4 cells of B0.8 mm. This can be seen in Figs. 6(b) and (c), which shows a higher magnification of the ytterbium silicates on the BN boundary phase and the Si3N4 cells in Fig. 6(a); the ytterbium silicates are the white flattened crystals in Figs. 6(a)–(c). XRD was performed before and after the oxidation testing for FMYB. The XRD analysis of the unoxidized FMYB sample revealed b-Si3N4, BN, and Yb4Si2O7N2 peaks, and b-Si3N4, BN, Yb4Si2O7N2, and Yb2Si2O7 peaks of 0 50 100 150 200 250 0.00 0.08 0.17 0.25 0.33 0.42 0.50 0.58 Stress [MPa] Crosshead Displacement [mm] Fig. 3. Flexural response of Si3N4/BN FMs with lanthanum oxide (FMLA). Fig. 4. Scanning electron micrograph of a side view of an FMLA flexural bar showing a shear-initiated fracture, between the outer and inner loading pins. Fig. 2. Scanning electron micrograph of a side view of an FMYB flexural bar showing a tensile initiated fracture. Fig. 5. (a) Lanthanum-rich crystals formed on the side surface of FMLA samples. (b) Magnification of the box in (a) of the crystals formed. May 2006 Rare-Earth Oxide Additives on Mechanical Properties and Oxidation Behavior 1617
1618 Journal of the American Ceramic Society-Karlsdottir and halloran Vol. 89. No 5 如=签 g,m,包m Fig. 6. (a) Oxidized surface of FMYB after 10 h at 1400.C in dry air. Large ytterbium silicates are apparent on the bn cell boundary phase;(b) magnification of the Bn boundary phase in(a) showing large ytterbium silicates; and(c)magnification of a Si3 N4 cell showing small ytterbium silicates on the surface Machined surface before oxidation used as sintering additives in the Si3 N4 phase the Sio, forms an SiaN O I\b2 For Sina with Y2O3 as a sintering additive,the oxide layer has been reported to consist of yttrium silicates (Y2Si2O,), which is the most stable compound in the Y-ShO ·Yb4S2O7N2 system21-23 Yb Si, O, skin has been reported to form on Si Na containing Yb2O3 as a sintering additive by a controlled oxida- tion process associated with the reaction between the Sio2 and Yb2O3. Yb2Si2O, has also been reported as one of the main Si3Na-SiC with Yb O3 as a sintering additive. nanocomposite oxidation products formed on the surface of Here, we have a 3D composite consisting of two different ce- ramic materials: Si3 Ng cells aligned in the uniaxial direction and Oxidized surface a bn cell boundary phase separating the Si3N4 cells. Si3N4 with Y2O3 and AlO3 as sintering additives have been shown oxidize at 1100%-1200C,.but bn starts oxidizing at lower temper atures; generally, the onset of measurable oxidation is about e|·Yb4s2OnN2 800C, but tends to decrease with higher oxygen impurity levels within the BN. At 800C the bn starts to oxidize into a liquid oxide(B,) by th tion reaction 2BN(s)+O2(g)=B2O3()+N2(g) The B,O3 liquid oxide is believed to start volatilizing at diffraction pattern of the surface of the FMY B before(a) temperatures above 1100.C The bn cell boundary phase in the FMYB sample consists of n grains surrounded by a glassy phase that migrates into the BI boundary phase during hot pressing. This can be seen from the oxidized FMYB sample; see Fig. 7. By comparing the two Fig 8, where the bright white spots represent the glassy phase diffraction patterns, it is evident that ther s considerable between the bn platelets, while the gray phase is the bn plate- formation of ytterbium silicates(Yb Si,O,)during the oxidation lets. After the oxidation and a decrease in the bn phase after the oxidation; see Fig. 7. an -100 um recess at the BN cell boundary while there was an It is well known that during oxidation of Si3 N4, a silica(SiO2) 4 um oxide layer on Si3N4 cells, and the oxidation zone was film is formed on the surface of the Si3 N4. If there are any oxides uniformly distributed around the surface; see Figs 9 and 10. 200 Fig 8. (a) Microstructure of the FMYB sample consisting of BN cell boundary phases and Si, N4 cells. (b) Magnification of the BN cell boundary, that consists of Bn grains surrounded by a glassy phase that migrated into the boundary phase during hot pressing
the oxidized FMYB sample; see Fig. 7. By comparing the two diffraction patterns, it is evident that there was considerable formation of ytterbium silicates (Yb2Si2O7) during the oxidation and a decrease in the BN phase after the oxidation; see Fig. 7. It is well known that during oxidation of Si3N4, a silica (SiO2) film is formed on the surface of the Si3N4. If there are any oxides used as sintering additives in the Si3N4 phase the SiO2 forms an oxide layer on the surface after reacting with the sintering additives.14,16,21 For Si3N4 with Y2O3 as a sintering additive, the oxide layer has been reported to consist of yttrium silicates (Y2Si2O7), which is the most stable compound in the Y–Si–O system.21–23 Yb2Si2O7 skin has been reported to form on Si3N4 containing Yb2O3 as a sintering additive by a controlled oxidation process associated with the reaction between the SiO2 and Yb2O3. 16 Yb2Si2O7 has also been reported as one of the main oxidation products formed on the surface of nanocomposite Si3N4–SiC with Yb2O3 as a sintering additive.17 Here, we have a 3D composite consisting of two different ceramic materials: Si3N4 cells aligned in the uniaxial direction and a BN cell boundary phase separating the Si3N4 cells. Si3N4 with Y2O3 and Al2O3 as sintering additives have been shown oxidize at 11001–12001C,21,23 but BN starts oxidizing at lower temperatures; generally, the onset of measurable oxidation is about 8001C, but tends to decrease with higher oxygen impurity levels within the BN. At 8001C the BN starts to oxidize into a liquid oxide (B2O3) by the oxidation reaction24,25: 2BNðsÞ þ 3 2 O2ðgÞ ¼ B2O3ðlÞ þ N2ðgÞ The B2O3 liquid oxide is believed to start volatilizing at temperatures above 11001C.26 The BN cell boundary phase in the FMYB sample consists of BN grains surrounded by a glassy phase that migrates into the boundary phase during hot pressing. This can be seen from Fig. 8, where the bright white spots represent the glassy phase between the BN platelets, while the gray phase is the BN platelets. After the oxidation testing on the FMYB sample, there was an B100 mm recess at the BN cell boundary while there was an B4 mm oxide layer on Si3N4 cells, and the oxidation zone was uniformly distributed around the surface; see Figs 9 and 10. 10 15 20 25 30 35 40 BN Intensity [Arb.unit] Intensity [Arb.unit] Oxidized surface 2θ 10 15 20 25 30 35 40 2θ Machined surface before oxidation Si3N4 BN Yb4Si2O7N2 Yb4Si2O7N2 Yb2Si2O7 Si3N4 (a) (b) Fig. 7. X-ray diffraction pattern of the surface of the FMYB before (a) and after (b) oxidation. Fig. 6. (a) Oxidized surface of FMYB after 10 h at 14001C in dry air. Large ytterbium silicates are apparent on the BN cell boundary phase; (b) magnification of the BN boundary phase in (a) showing large ytterbium silicates; and (c) magnification of a Si3N4 cell showing small ytterbium silicates on the surface. Fig. 8. (a) Microstructure of the FMYB sample consisting of BN cell boundary phases and Si3N4 cells. (b) Magnification of the BN cell boundary, that consists of BN grains surrounded by a glassy phase that migrated into the boundary phase during hot pressing. 1618 Journal of the American Ceramic Society—Karlsdottir and Halloran Vol. 89, No. 5
May 2006 Rare-Earth Oxide Additives on Mechanical Properties and Oxidation Behavior Unaffected Interior The reason why larger Yb,, were found in the bn cell boundary than on the surface of the Si3N4 cells is that the 100 um recess BaTION boro-silicate glass in the boundary phase has a lower viscosit at bN cell ZONE than the glass in the grain-boundary phases, thus allowing more pid diffusion and thus larger Yb Si,O, grains emanated from the bn boundary phases than from the Si3N4 cells. Figure 1 l shows a schematic diagram of the oxidation mecha- 4 um oxide nism of the fmB layer on Si, cells IV. Conclusion The Si3 N4/BN FM with 8 wt% Yb2O3 showed a flexural strength of 340+91 MPa and graceful failure with a WoF of 2126+940 J/m". These properties are comparable with reported 0k50 100x Si3N4/BN FMs with 6 wt%Y2O3 and 2 wt% AL,O3. The ox- idation test of the FMs with Yb,O3 revealed a thin oxide scale Fig 9. FMYB sample after oxidation at 1400C for 10 h. The oxida- containing small Yb Si,O, on the Si3N4 cells but large Yb2Si,O tion zone was uniformly distributed around the surface on the Bn cell boundary. Also, microscopic analysis after the oxidation test showed an 100 um recess in the Bn cell bound- ry and an 4 um oxide scale on Si3 N4 cells, uniformly dis- tributed around the surface of the sample The Si3N4/BN FMs with La2O3 showed a flexural strength of 298+90 MPa and brittle failure in the majority of the samples Si The brittle behavior and low strength of the FMLA samples "Glassy" phase compared with the FMYB samples were believed to be due to disintegration of the sample, because of the hydration of La2O3 from the rare-earth apatite phase, LasSi3O12N. The surface of the FMLa sample after oxidation for 10 h at 1400.C in dry air showed severe oxidation In conclusion, the Si3N4/BN FM with Yb, O3 as a sinterin additive has proven to be similar in strength at room tempe ture to the commercially available Si3N4/BN FMs with 6 wt% Y,O3 and 2 wt% AlO,, and to have a promising oxidation be- havior. The rare-earth sintering aid, La2O3, has shown not to confer suitable mechanical properties or oxidation resistance to References b Si O 'S. Baskaran, S.D. Nunn, D. Popovic, and J. W. Halloran, "Fibrous Mono- Penetration in BN cell bound Ceramics,J. Am. Ceran. Soc., 80[10]2471-87(1997) R. w. Trice and J. w. Halloran, "Elevated-Temperature Mechanical Proper- ties of Silicon Nitride/ Boron Nitride Fibrous Monolithic Ceramics. Fig 10. Side view of a cross-section of an FMYB sample showing the Soc,832]31l-6(2000) Bn boundary phase after oxidation Trice and J. w. Halloran, " Enect of Sintering Aid Composition on the g of Si3 Na/BN Fibrous Monolithic Ceramics. J. Am. Ceram. Soc., 82 Thus, the BN cell boundaries started to oxidize before asts Monolithic ces haus Mim,H用时(m上m过 R. W. Trice and J. w. Halloran. "Influen the Interfacial Fracture Energy of Silicon Nitride/ Boron Nitride Fibrous into liquid oxide (b2O3). The B2O3 liquid oxide then started t platize at higher temperatures and yielded Yb2Si2O7 and left behind some residue of the amorphous glassy phase; see Fig 10 C. Goretta, F. Gutierrez-Mora N. Chen, J. L. Routbort, T A. Orlova, B I. M. Y. He, D. Singh, J. C. McNulty, and F. w. Zok, Thermal Expansion tional and Cross-Ply Fibrous Monoliths, Compos. Sci. Technol 1400°C.d ry a Clarke and G. Thomas. "Grain Boundary in a Hot-Pressed Mgo Fluxed J. Am. Ceram. Soc. 60 491-5(1977) ID. R. Clarke, " On the Equilibrium Thickness of Intergranular Glass Phases in Ceramic Materials. " J. Am. Ceran. Soc. 70[1] 15-22(1987). Y. Goto and G. Thomas. ""Microstructure of Silicon Nitride Co d with Rare-Earth o Growing SiO2 film semat-Nalsve Yb2Si207 glassy Is,and s M. Johnson. "Strength and Creep Be ase borosilicate havior of Rare-Earth Disilicate-Silicon Nitride Ceramics, J. Am. Ceram. Soc 8]2050-5(1992). Cinibulk, G. Thomas, and S. M. Johnson. "" Oxidation Behat of Rare-Earth Disilicate-Silicon Nitride Ceramics, J. Am. Ceran. Soc., 75[81 IH. Park. H.-E. Kim, and K. Niihara. ""Microstructural Evolution and Me Fig. 11. Schematic of the oxidation process for the exposed Si3N4 cells chanical Properties of SiN4 with Yb2O3 as Sintering Additive, "J. Am. Ceram and bn boundary phase Soc,803750-6(1997)
Thus, the BN cell boundaries started to oxidize before the Si3N4 cells, where the BN grains in the cell boundary started to oxidize into liquid oxide (B2O3). The B2O3 liquid oxide then started to volatize at higher temperatures and yielded Yb2Si2O7 and left behind some residue of the amorphous glassy phase; see Fig. 10. The reason why larger Yb2Si2O7 were found in the BN cell boundary than on the surface of the Si3N4 cells is that the boro-silicate glass in the boundary phase has a lower viscosity than the glass in the grain–boundary phases, thus allowing more rapid diffusion and thus larger Yb2Si2O7 grains emanated from the BN boundary phases than from the Si3N4 cells. Figure 11 shows a schematic diagram of the oxidation mechanism of the FMYB. IV. Conclusion The Si3N4/BN FM with 8 wt% Yb2O3 showed a flexural strength of 340791 MPa and graceful failure with a WOF of 21267940 J/m2 . These properties are comparable with reported Si3N4/BN FMs with 6 wt% Y2O3 and 2 wt% Al2O3. The oxidation test of the FMs with Yb2O3 revealed a thin oxide scale containing small Yb2Si2O7 on the Si3N4 cells but large Yb2Si2O7 on the BN cell boundary. Also, microscopic analysis after the oxidation test showed an B100 mm recess in the BN cell boundary and an B4 mm oxide scale on Si3N4 cells, uniformly distributed around the surface of the sample. The Si3N4/BN FMs with La2O3 showed a flexural strength of 298790 MPa and brittle failure in the majority of the samples. The brittle behavior and low strength of the FMLA samples compared with the FMYB samples were believed to be due to disintegration of the sample, because of the hydration of La2O3 from the rare-earth apatite phase, La5Si3O12N. The surface of the FMLA sample after oxidation for 10 h at 14001C in dry air showed severe oxidation. In conclusion, the Si3N4/BN FM with Yb2O3 as a sintering additive has proven to be similar in strength at room temperature to the commercially available Si3N4/BN FMs with 6 wt% Y2O3 and 2 wt% Al2O3, and to have a promising oxidation behavior. The rare-earth sintering aid, La2O3, has shown not to confer suitable mechanical properties or oxidation resistance to Si3N4/BN FMs. References 1 S. Baskaran, S. D. Nunn, D. Popovic, and J. W. Halloran, ‘‘Fibrous Monolithic Ceramics: I, Fabrication, Microstructure, and Indentation Behavior,’’ J. Am. Ceram. Soc., 76 [9] 2209–16 (1993). 2 D. Kovar, B. H. King, R. W. Trice, and J. W. Halloran, ‘‘Fibrous Monolithic Ceramics,’’ J. Am. Ceram. Soc., 80 [10] 2471–87 (1997). 3 R. W. Trice and J. W. Halloran, ‘‘Elevated-Temperature Mechanical Properties of Silicon Nitride/Boron Nitride Fibrous Monolithic Ceramics,’’ J. Am. Ceram. Soc., 83 [2] 311–6 (2000). 4 R. W. Trice and J. W. Halloran, ‘‘Effect of Sintering Aid Composition on the Processing of Si3N4/BN Fibrous Monolithic Ceramics,’’ J. Am. Ceram. Soc., 82 [11] 2943–7 (1999). 5 R. W. Trice and J. W. Halloran, ‘‘Influences of Microstructure and Temperature on the Interfacial Fracture Energy of Silicon Nitride/Boron Nitride Fibrous Monolithic Ceramics,’’ J. Am. Ceram. Soc., 82 [9] 2502–8 (1999). 6 Y.-H. Koh, H.-W. Kim, H.-E. Kim, and J. W. Halloran, ‘‘Thermal Shock Resistance of Fibrous Monolithic Si3N4/BN Ceramics,’’ J. Eur. Ceram. Soc., 24, 2339–47 (2004). 7 K. C. Goretta, F. Gutierrez-Mora, N. Chen, J. L. Routbort, T. A. Orlova, B. I. Smirnov, and A. R. de Arellano-Lope´z, ‘‘Solid-Particle Erosion and Strength Degradation of Si3N4/BN Fibrous Monoliths,’’ Wear, 256, 233–42 (2004). 8 M. Y. He, D. Singh, J. C. McNulty, and F. W. Zok, ‘‘Thermal Expansion of Unidirectional and Cross-Ply Fibrous Monoliths,’’ Compos. Sci. Technol., 62, 967–76 (2002). 9 D. R. Clarke and G. Thomas, ‘‘Grain Boundary in a Hot-Pressed MgO Fluxed Silicon Nitride,’’ J. Am. Ceram. Soc., 60 [11–12] 491–5 (1977). 10D. R. Clarke, ‘‘On the Equilibrium Thickness of Intergranular Glass Phases in Ceramic Materials,’’ J. Am. Ceram. Soc., 70 [1] 15–22 (1987). 11Y. Goto and G. Thomas, ‘‘Microstructure of Silicon Nitride Ceramics Sintered with Rare-Earth Oxides,’’ Acta Metall. Mater., 43 [3] 923–30 (1995). 12M. Liu and S. Nemat-Nasser, ‘‘The Microstructure and Boundary Phases of In-Situ Reinforced Silicon Nitride,’’ Mater. Sci. Eng., A254, 242–52 (1998). 13M. K. Cinibulk, G. Thomas, and S. M. Johnson, ‘‘Strength and Creep Behavior of Rare-Earth Disilicate–Silicon Nitride Ceramics,’’ J. Am. Ceram. Soc., 75 [8] 2050–5 (1992). 14M. K. Cinibulk, G. Thomas, and S. M. Johnson, ‘‘Oxidation Behavior of Rare-Earth Disilicate–Silicon Nitride Ceramics,’’ J. Am. Ceram. Soc., 75 [8] 2044–9 (1992). 15H. Park, H.-E. Kim, and K. Niihara, ‘‘Microstructural Evolution and Mechanical Properties of Si3N4 with Yb2O3 as Sintering Additive,’’ J. Am. Ceram. Soc., 80 [3] 750–6 (1997). Fig. 9. FMYB sample after oxidation at 14001C for 10 h. The oxidation zone was uniformly distributed around the surface. Fig. 10. Side view of a cross-section of an FMYB sample showing the BN boundary phase after oxidation. 1400°C, dry air B2O3 volatilizing Si3N4 Si3N4 Growing SiO2 film Yb2Si2O7 + glassy phase + borosilicate Intact BN Fig. 11. Schematic of the oxidation process for the exposed Si3N4 cells and BN boundary phase. May 2006 Rare-Earth Oxide Additives on Mechanical Properties and Oxidation Behavior 1619
Journal of the American Ceramic Society-Karlsdottir and halloran ol.89,No.5 l6s. K. Lee and M. J. Readey, "Development of a Self-Forming Ytterbium Si- -FF. Lange. S. C. Singhal, and R. C. Kuznicki, ""Phase Relations and stability icate Skin on Silicon Nitride by Controlled Oxidation.J.A. Ceram. Soc., 85 [6 Studies in the Si3NFSiOrY,O3 Pseudot System. J. Am. Ceram. Soc. 60 5624952(1977) H. Park, H -W. Kim and H -E. Kim. ""Oxidation and Strength Retention 'A. Bellosi. G.N. Babini. L. P. Huang and X.R. Fu.""Phase Efects Monolithic Si3 Na and Nanoc Oxidation Resistance in Si3NrAL-OrY-O3- hem. phus Ss.S. Chan and A. T. Bell, "Characterization of the Preparation of Pd/Sioz 2N. S. Jacobson and G. N. Morscher, "High-Temperature Oxidation of and PaLaz o, by Laser Raman Spectroscopy,Catal. 89, 433-41(1984) Boron Nitride: I, Monolithic Boron Nitride, J. Am. Ceram. Soc. 82 [ 2]393-8 2N. S. Jacobson and G N. Morscher. "High-Temperature Oxidation of Boron H Klemm, C. Taut, and G. Wotting, ""Long-Term Stability of Nonoxide Ce- Nitride: Il, Boron Nitride Layers in Composites, " J. Am. Ceram Soc., 82 [6]1473- (s. p Taguchi nd S. Ribeiro, slicon Nitide Oxidation Behavio at norc 82 Nitride-A New Approach aceous Structures, "Carbon, 33 [41 389-95(1995)
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