Sic (FCF'40 NLC) as received

Transcription

1 CHAPTER 3 RESULTS The various steps involved in this work can be described as a) coating of Sic particles b) sol gel synthesis of alumina-sic composite c) densification and microstructure development and d) mechanical characterisation. 3.1 COATING OF Sic ACTIVATION TREATMENT As received Sic powders are generally associated with minor carbon and other organic contamination as a result of which a heat treatment in air at temperatures o,@j Sic (FCF'40 NLC) as received ,s 200 ' 3b ' 4b ' 5b ' 6bO ' 7b ' 8!)0 Temperature "C Fig 3.1 Thermal analysis data of as received Sic powders

2 around 650 C is necessary for burnout. Moreover, the presence of such impurities promote demixing of Sic powders in aqueous suspensions. Fig 3.1 shows the thermal analysis of Sic powders in air up to 700 C. There is an initial weight gain up to 50 C which could be an artefact or moisture adsorption. This is followed by a small weight loss due to evaporation of adsorbed water. Thereafter is a considerable weight gain amounting to 1% initial. This is initiated at temperatures as low as 300 C. The FTIR pattern of the as received Sic powder shown in Fig 3.2 indicate the characteristic peak of Sic corresponding to u Si-C mode to appear around 850 cm-'. There is a broad band around 1070 cm-' and is assigned to the silica impurity associated with powders. On thermal activation (Fig 3.3) at 650 C / 30 min distinct peaks appear at wave numbers of 1250, 1070 and cm-' and are characteristic of amorphous silica'28. Fig 3.4 provides the FTIR pattern of Sic powders after chemical leaching with 10% Hydrofluoric acid. All the peaks corresponding to amorphous silica are absent in this pattern. O t -- r la Wave numbers cm- Fig. 3.2 F'TIR pattern of as received Sic powders

3 - morphous silica amorphoussilica - - u Si-C Wave numbers cm-1 Fig 3.3 FTIR pattern of Sic powders heat treated at 650 C/30 min '20~*bv.ted(TherrnaHy treated and HF leached) Wave number cm-' Fig 3.4 FTIR pattern of Sic: powders after thermal activation and HF leaching.

4 3.1.2 COATlNG a. FTIR Fig 3.5 represents the spectrum of Sic powder coated with 25 wt% alumina in the gel form after drying. Characteristic peak of Sic remain unchanged at 100 adsorbed water adsorbed water 20 u SIC a) gel! 1 0 t I Wave numbers cm-' Fig 3.5 FTIR pattern of Sic-25 wt% alumina powders (gel) 100 water adsorbed water 40 b) calcined Wave numbers cm-' Fig 3.6 FTIR pattern of Sic-25 wt% alumina powders calcined at 500 C

5 850 cm-'. The strong bands at 3430 cm-' and 1630 cm-i correspond to the stretching and bending modes respectively of adsorbed water. The broad band at cm-' corresponds to Al-0 stretching vibrations associated with an octahedrally co-ordinated Al, A The FTIR pattern of coated powder calcined at 500 C presented in Fig 3.6 more or less resembles that of the gel except for the reduction in intensity of the band at 600 cm-i. b. BET Specific Surface Area The surface area values of the coated powders after drying and after calcination at 500 C I 2h are presented in Fig 3.7. There is a progressive increase in the surface area values with increasing weight fraction of alumina both in the gel and in the calcined powders. Fig 3.7 Plot of surface area values with increasing alumina weight fraction

6 Pig 3.8a Adsorption isotherms of Sic powders coated with varying weight fractions of alumina ( gel samples) Fig 3.8b Adsorption isotherms of coated Sic powders calcined at 500 C The adsorption isotherms of the coated samples before and after calcination are presented in Fig 3.8a and Fig 3.8b. The uncoated samples have an adsorption behaviour of type 11 characteristic of non porous solids. The samples with lower alumina fractions have similar adsorptions as that of the uncoated Sic. A detailed

7 analysis of the t-plots( Fig 3.9a and 3.9 b) indicate that the coated powders with up to 15 wt% alumina are similar with a straight line plot while those with higher concentrations have two straight lines with different slope. This is an indication of mesoporosity and type IV adsorption behaviour Fig 3.9a t-plot analysis of coated Sic powders before calcination Fig 3.9b t-plot analysis of coated Sic powders calcined at 500 C

8 c. Transmission Electron Microscopy Morphology of coated and uncoated powders as observed by TEM are presented in Fig 3.10 (a-d). Uncoated Sic particles are irregular shaped with sharp edges and has an average particle size around x1 ( Fig 3.10a). Sic particles coated with 5 wt% alumina and calcined at 500 C zre shown in Fig 3.10b. The nature of the coating.phase reveals that of a poorly crystalline material adhering firmly to the SIC surfaces. Thickness of the coating is non uniform though it could be observed that available surface is all covered with the precipitated phase, as in Sic-15wt% alumina (Fig 3.10~). With increasing amounts of the coating phase there is a tendency of the coating particles to bridge and bind other coated particles leading to formation of agglomerates as observed in Sic-25 wt% alumina (Fig 3.10d). Fig 3.10 TEM picture of a) Uncoated Sic 58

9 Fig 3.10b TEM picture of Sic-5 wt% alumina calcined at 500 C Fig 3.10 TEM picture of c) Sic- 15 wt% alumina calcined at 500 C d) Sic-25 wt% alumina calcined at 500 C

10 Fig shows the TEM of silicon carbide particles coated with 25 wt% of alumina in the gel form. Individual Sic particles are seen with the precipitated boehmite adhering to its surfaces. Fig 3.11 TEM picture of Sic particles coated with 25 wt% alumina ( gel) d. Zeta Potential Measurements Formation of alumina coatings on Sic is further supported by zeta potential measurements shown in Fig which plots the zeta potentials of SIC particles with various alumina concentrations compared with that of uncoated SIC and pure alumina. There is a clear shift of the isoelectric point (IEP) of SIC particles with increasing alumina fraction. The isoelectric point of uncoated particles is around ph 4 and that of alumina particles is at ph 9. As the concentration of alumina increases the IEP shifts towards higher ph valucs, indicating that the Sic particles are getting

11 coated with fine alumina prt:cipitates. The IEP of Sic -25 wt % alumina approaches that of pure alumina as shown by the zeta potential values with increasing ph of the suspension. Pig Zeta Potential as a function of ph for Sic particles coated with alumina compared with that of uncoated Sic and pure alumina

12 3.2 PROCESSING OF COMPOSITES THERMAL ANALYSIS Thermal analysis of the boehmite powder used as the sol gel precursor for alumina in the processing of composites is presented in Fig The TG pattern indicates two regions of mass loss corresponding to - 22 % within temperatures less than 600 C. DTA plot indicates two endothermic peaks at 113OC and 455.4"C and an exothermic peak at 1220 C. 111 the case of boehmite seeded with 2 wt.% a alumina Fig 3.13 Thermal analysis pattern of as received boehmite powder

13 Fig 3.14 Thermal analysis of hoehmite gel seeded with 2 wt.% a- alumina seeds Fig Thermal analysis pattern of alumina-5 vol% Sic composite precursor seeded with 2 wt.% a-alumina seeds

14 seeds the TG pattern remains the same while the final exothermic peak in DTA occurs at 80 C lower than unseeded boehmite powders (Fig 3.14). In the case of alumina-5 vol% SIC composite precursor gel seeded with 2 wt.% a-alumina the final exothermic peak is at a temperature of "C. In addition to that a small but, broad peak is also seen at temperatures around 300 C ( Fig 3.15) PHASE FORMATION Fig 3.16 presents the phase transformation of the alumina-5 vol% Sic composite precursor gel seeded with 2% a-alumina. Precursor gel after drying at degree Fig 3.16 XRD pattern of the phases formed on calcination of alumina-5 vol% Sic seeded precursor at different temperatures (y, 6, 0 and a denotes the corresponding alumina phases)

15 75 C mainly shows AlOOH. (In calcination at 700 C alumina is mainly y-alumina phase. A further increase of 100 C shows the transformation to &phase which remain more or less unchanged at 900 C. At 1000 C the composition is a mixture of 6, 8 transitional aluminas and a -alumina. The phase formation with temperature is summarised as in table 3.1 Table 3.1. Formation of alumina phases on calcination - Temperature 7S C 700 C: 800 C' 9000~- Major Alumina Phases AlOOH (Boehmite) y-alumina &alumina &alumina and traces ~~7 6,0 and a-aluminas DEPENDENCE OF CALCINATION CONDITIONS ON DENSIFICATION Fig 3.17a shows the green density values of sol gel precursors after calcination at the said temperatures, followed by milling and compaction. There is not much of a significant difference in the green density values. However, there is a marked difference on the sintered density values where a precursor calcined at 1000 C for 2h under nitrogen shows maximum densification (99.2%TD) and the density values shows an Increase with increasing calcination temperature.

16 Calcination Tempemhue OC Fig. 3.17a Dependence of calcination temperature on green density (b) 99 - F $-.&' 98 - Ti 'i m /= Sitered at 1700/90 min Ad80 bar W Calcination temperature C Fig. 3.17b Dependence of calcination temperature on sintered density

17 3.2.4 DILATOMETER STUDIES m Tie Fig. 3.18a Shrinkage profile of sol gel composite precursor calcined at 1000 C U O 5-5, r XI Time (min) Fig. 3.18b Shrinkage profile of sol gel composite precursor calcined at 900 C Figs (a-c) shows the linear shrinkage of samples with temperature and time on heating to 1700 C 1 90 min in nitrogen. The sol gel composite precursor calcined at 1000 C shows a two stage shrinkage curve with the former having an onset at -looo C and the later at 1300 C ( Fig 3.18a). The total linear shrinkage corresponds

18 ~ p.7.-.-tp-.. to 22% initial with the first stage having 9% and the final stage 13%. The shrinkage profiles of the precursors calcined at 900 (Fig. 3.19b) and at 800 C (Fig. 3.19~) have higher shrinkage during the first stage. Time (rnin) Fig. 3.1% Shrinkage profile of sol gel composite precursor calcined at 800 C. alumina-5vol%sic composite ' adsorbed water 40 V adsorbed water 20 USi-C. A104 7 r Y 4W 9V0 1*1VO Wave numbers cm- 1 Fig l0oo0c FTIR pattern of alumina-sic composite precursor calcined at

19 FTIR pattern of the composite precursor calcined at 1000 C is shown in Fig In addition to peaks corresponding to adsorbed water and Sic, bands are appearing in the range cm.' and in the range cm-l. The former is assigned to u- A104 and the later to A106 corresponding to alumina phases present at that temperature of calcination COMPACTION BEHAVIOUR Fig 3.20 shows the variation in green and sintered densities of alumina - 5 vol% Sic composite precursors calcined at 1000 C with CIP pressure. The green density values increase from 41.5 % TD to %TD on increasing the cold isostatic pressure from 200 MPa to 500 MPa. This effect has its reflectance on the sintered density values also where an increase from 93% to above 98% TD has been obtained with increase in CIP pressure. CIP Pressure (MPa) Fig 3.20 Effect of CIP pressure on green and sintered densities.

20 3.3EFFECT OF SEEDING ON PHASE TRANSFORMATION The influence of a -alumina seeds on the a-alumina phase transformation temperature was studied by DTA. Fig 3.21 shows the plot of a-alumina formation wt%seeds Fig 3.21 Variation of a-alumina formation temperatures with amounts of seed temperature with % weights of a-alumina seeds. An unseeded composite precursor gel shows the a-alumina formation temperature at 1196OC. Addition of 1% a- alumina seed lowers the tbrmation temperature to 1167 C and further down to 1159 C on increasing the concentration to 2%. There is very little change in the a- alumina formation temperatures with further increase up to 5% seed ON DENSIFICATION The densification behaviour of seeded and unseeded composites on sintenng from 155OoC to 1700 C has been studied. Fig 3.22 shows the piot of temperature versus sintered density (%, theoretical). The seeded composites are shown to have improved densification over the unseeded counterhart over the range of temperatures

21 investigated. Moreover there i:; no further improvement of sintered density from 96% TD for unseeded samples even after increasing the temperature to 1700 C for 90min. Fig Sintered density with temperature for seeded and unseeded samples DENSIFICATION AND MICROSTRUCTURAL DEVELOPMENT Fig 3.23 compares the densification data of sol gel derived alumina and -,.,.,.,.,.,., M) M) 1750 Sintering Tenyemhue "C Fig 3.23 Densification data of monolithic alumina and nanocomposite

22 alumina-5 vol% Sic, both seeded with 2% a alumina seeds. Monolithic alumina attains 99% TD at temperatures of 1550 C. However, the composite samples require 1700 C for the same amount of densification as in alumina. Microstructural analysis of the sintered alumina (Fig 3.24) shows grains with an average size of 5 pm. Fig 3.24 SEM picture of polished and thermally etched monolithic alumina Composite samples sintered at 1550 C show (Figs 3.25a and 3.25b) closed porosity spread through out the sample. The average grain size is under 1 pm. Fig 3.25a SEM picture of alumina-5 vol% Sic sintered at 1550 C/1h

23 Fig 3.251, SEM picture of alumina-5 voloa Sic sintered at 1550 C/lh (higher magnification) The pore size is around 500 nm. On increasing the sintering temperature to 1650 C the samples (Figs. 3.26a 3.26b) attained good densification and there is homogeneous distribution of Sic particles within grains and along grain boundaries. Fig 3.26a SEM micrograph of alumina-5vol0/0sic nanocomposite sintered at 1650 C

24 'I'his is more clear in an AFM micrograph presented in Fig 3.27a at lower magnifications and Fig 3.27b at higher magnifications. Fig 3.26b. SEM micrograph of alumina-5 vol% Sic sintered at 1650 C 0 2D.Uvn un Data type Clnpl i tude Data type Clnpl i tude 2 range 70.0 nn 2 range 30.0 nm (a) ('4 Fig 3.27 AFM picture of an alumina- 5~01% Sic nanocomposite sintered at 165O0C/lh, Ar (a) lower magnification and (b) higher magnification

25 Figs 3.28~1 and 3.28b shows the SEM micrograph of conlposite san~ple sintered at 1700 C/90 min. Grains have grown considerably bigger but there is excellent dispersion of Sic particles through out. Pig 3.28a SEM picture of alumina-5 vol%sic sintered at 1700 C/90min Fig 3.28b SEM picture of alumina-5 vol% Sic sintcred ;it 1700/90 min

26 The irregular grain boundaries, typical of a nanocomposite microstructure is also clearly visible. The AFM pictures prcsented in Fig 3.29 (a and b) shows an alumina matrix grain having Sic particles within and along its boundaries pu!j 5.00 pu Data type hnpl i tude Data type Rwpl i tude Z range 70.0 nu Z range 30.0 nu (a) (b> Fig AFM picture of an aiumina-5vo1 h Sic nanocomposite sintered at 1700 C/lh, Ar a) lower magnification b) higher magnification 3.4 FRACTURE MODE There is a significant difference in the fracture behaviour between monolithic alumina and alumina-sic nanocomposites. The fracture mode, in alumina, as presented in Fig 3.30~1, indicates that of a inter granular fracture while that of nanocomposite clearly shows predominantly transgranular fracture ( Fig 3.30b).

27 Fig 3.30a Fracture surface of sintered monolithic alumina Fig 3.30b Fracture surface of sintered alumina-5 vol0/0 SiC nanocomposite

28 3.5 MICROSTRUCTURE DEVELOPMENT IN UNSEEDED NANOCOMPOSITES Fig 3.31 (a-c ) show the microstructures of unseeded composite samples sintered at 1550 C, 1650 C and 1700 C respectively. The unseeded composite sample at 1550 C shows distributed porosity mainly along the boundaries and at triple points. Fig 3.3La SEM picture of an unseeded sample sintered at 1550 C/60min Fig 3.31 b SEM picture of an unseedcd snmplc sintcred at 1650 C/60min

29 On sintering at 1650 C the grail~s I~ave grown larger- will1 Sic particles segregating at grain bo~rndaries and triple points. This is more clear on samples sintered at 1700 C. Fig. 3.31~ SEM picture of an unseeded sample sintered at 1700 C/90 min 3.6 MECHANICAL PROPERTIES Fig 3.32 present the four point bending strengths of alumina-5 vol% Sic seeded with 2 % a-alumina seeds with sintering temperature and density. A maximum value of 63OMPa is achieved for samples sintered at 1700 C and having 99.2%TD.There is an increase in fracture strength with increasing sintering temperature on account of the improved densification attained. Table 3.2 compares the mechanical properties of alumina with nanocomposites.

30 Fig3.32 Four point bend strength values of alumina-5 vol% Sic composites with sintering temperature Table 3.2 Comparison of mechanical data for alumina and nanocomposite Property Alumina Nanocomposite Bending Strength (Four point) MPa Fracture Toughness (Indentation Strength in Bending) MPa Jm Hardness (GPa)

31 3.7 COMPARISON OF PROCESSING METHODS Alumina-5 vol% Sic composites prepared by involving sol gel coated precursors are compared with those prepared by the conventional solid state mixing of the constituent powders of a -alumina and Sic. Alternatively transitional alumina obtained by the calcination of boehmite at 1000 C was also used as the alumina source in a separate experiment. Two types of a-alumina powders were employed. High purity tamei (TM-DAR) alumina powders and Condea HPA 0.5 powders $ degree Fig XRD patterns of the composite precursors Fig 3.33 compare the XRD pattern of the three composite precursors. The mixture of a-alumina anti Sic is characterised by well defined peaks of the

32 respective constituents. 'l'he transitional alumina - Sic sample is mainly a mixture of 6 and 0 transition alumina phases along with Sic. The sol gel coated precursor shows a -alumina peaks also in addition to the transition alumina phases. Fig 3.34 compares the densification profiles of the three precursors on heating to 170O0C/90 min in nitrogen atmosphere. The a-alumina + Sic mixture shows a linear sintering shrinkage of 1'7% initial. The onset of the shrinkage curve is at - 13OO0C. Fig 3.34a Shrinkage profile of a-alumina + SIC mixture The sol gel coated precursors shows a two stage shrinkage curve with the former having an onset at 1000 C and the later at-1300 C. The total linear shrinkage corresponds to 22% initial. On the contrary the transitional alumina / Sic mixture, though shrunk in two stages, the final shrinkage rate is exceptionally slow. Moreover, the total linear shrinkage is only 20% initial.

33 Time Fig 3.34b Shrinkage profile of sol gel composite precursor calcined at 1000 C Y : T,.".,,lo".l".l".+ LC + - $ 1000 a $ r. 2 % -10 V1 Z 3 h -15 %(u 1 ~ [ b 1 5oo s Time (min) Fig 3.34~ Shrinkage profile of transition alumina + Sic: composite precursor

34 Fig 3.35a compares the green density values of the precursors. The a-alumindsic composition is having 58% theoretical density on green compaction while the sol gel coated precursors and transition alumina /Sic samples attain only 48% TD under d& Tdcn dvrira tam AuriwSC tamei Sol gel Transition +Sc alumina+sic precursor alumina+sic (a> Ib) Fig 3.35 Variation in densities of composite precursors a) green b) On sintering at 1700 C 190min identical conditions of preparation. However, on sintefing at 1700 C/90 min under argon atmosphere both a-alurninalsic mixture and sol gel coated precursor attained nearly the same densification levels of >99% TD but the transition aluminajsic mixture could reach only 95% TD. The two a-alumina powders used ( tarnei, TM- DAR and Condea HPa 0.5) showed more or less similar densification characteristics with final densities around 99% TD.

35 3.7INFLUENCE OF MgO ADDITION The effect of MgO dopant on the densification and microstructure development of alumina-5~01% Sic composite was studied on samples doped with 1 wt% MgO introduced during the preparation of the composite as nitrate salt solution. Fig compares the densification behaviour of such composites with the undoped samples. Table 3.3 compares the grain size values of the sintered composites with and without MgO. Samples sintered with MgO dopant are shown to have undergone exaggerated grain growth with grain sizes growing upto 100pm with increase in sintering temperature /' 3 %- 95 * 1450 I Sitering Temperature O C Fig Densification behaviour of MgO doped and Undoped Nanocomposites

36 Table 3.3 Variation in grain sizcs with sintering temperature for I wt'% MgO doped and undoped nanocomposite Sintering Temperature C 1450 C 1550 C 1650 C 1700 C Undoped - <1 pm 2 CLm 4 Pm lwt% MgO Doped 1-2 pm pm pm - Figures 3.37 a and b compare the AFM micrographs of MgO doped alumina and pure alumina sintered under identical conditions. The doped alumina shows fine, equiaxed and uniform grained microstructure while that of undoped alumina shows long elongated grains associated with discontinuous grain growth occurring at final ~JM Data type Anpl i tude 2 range 200 nw Fig. 3.37a AFM picture of undoped alumina

37 un Data type Avpl i tude Z range 20,O nn Fig. 3.37b AFM Micrograph of MgO doped alumina stages of sintering. Microstructures of the chemically etched samples are presented in Fig 3.38 (a-c). At 1450 C the microstructure is porous with fine grain sizes. Fig. 3.38a SEM Microstructure of chemically etched alumina-5 vol% SIC doped with lwt% MgO sintered at 1450 C

38 Fig. 3.38b SEM Microstructure of chemically etched alumina5 vol% SiC doped with lwt% MgO sintered at 1550 C (bar-20pm) The samples sintered at 1550 C shows grain sizes one order of magnitude higher than that at 1450 C within a range of pm. The microstructure of sample sintered at C shows abnormally grown grains typical of glassy phase sintering. Fig. 3.38~ SEM Microstructure of chemically etched alumina-5 volo/o Sic doped with 1wt0/o MgO sintered at 1650 C