Densification, characterization and oxidation studies of (TiCr)B2+20% MoSi2

io

u

Oxidation studies

ts oer an frOx750one/ccnesfc

7.510 K from room temperature to 900 C and electrical resistivity of the composite as 2.1, 2.6 and3.0 cm at 25, 500 and 1000 C respectively. Observed nature of oxidation was parabolic for all composites.

thick glassy lm was observed with increased proportion of CrB2 content in the

tematewear as highd electrof strenl rod a

nuclear reactors due to the ability of boron to a

consolidation, poor fracture toughness and oxidation resistance restrict sinter additive would be a material with highmelting point and thermal

Int. Journal of Refractory Metals and Hard Materials 37 (2013) 1228

Contents lists available at SciVerse ScienceDirect

Int. Journal of Refractory M

j ourna l homepage: www.e lsthe widespread use of TiB2 [2].Monolithic TiB2 can be densied only at high temperatures, with the

application of pressure during sintering. However, high temperaturescause abnormal grain growth and microcracks, which are detrimental tothe mechanical properties. Densication of TiB2 is feasible at lower tem-peratures using sinter additives such as metals (Fe, Ni, Cr, Co, Cu, etc.)[35] and ceramics (oxides, nitrides, carbides, borides, silicides) [3,6

conductivity, similar coefcient of thermal expansion (CTE) and a suit-able protective oxide former. Formation of a protective oxide layer ofAl2O3, SiO2 or complex oxideswith B2O3 and/or TiO2 has been attemptedfor improving the oxidation resistance.

CrB2 is a potential additive for high performance materials like car-bides and borides [15], due to its high melting point, high strength atelevated temperatures, oxidation resistance, high temperature wear re-12]. The presence of metallic additives degradproperties and hence is not considered for high

Corresponding author. Tel.: +91 22 2559 3937; faxE-mail address: [email protected] (T.S.R.C. Murthy

0263-4368/$ see front matter 2012 Elsevier Ltd. Allhttp://dx.doi.org/10.1016/j.ijrmhm.2012.10.006bsorb fast and slow neu-. However, difculties in

layer of the additive is known to form a glassy phase, thereby deteriorat-ing the mechanical properties of TiB2. The most appropriate choice as atrons without forming long lived radionuclide1. Introduction

Titaniumdiboride (TiB2) is a candidaapplications, cutting tools, heavy dutyelectrodes in metal smelting due to ithigh hardness and wear resistance, gootivity, chemical stability and retentiontures[13]. TiB2 is also used as controfor the improved oxidation resistance of this composite. 2012 Elsevier Ltd. All rights reserved.

rial for high temperaturepplications, armour andmelting point (3225 C),ical and thermal conduc-gth at elevated tempera-nd shielding material in

Fully dense TiB2 has been fabricatedwith the addition of ceramics such asB4C, Si3N4, SiC, AlN, TaC,MoSi2, TiSi2 and TaN by hot pressing [3]. Additionof CrB2 with or without other additives in densication by conventional/microwave sintering or as pre-alloyed with TiB2 in hot pressing hasbeen found to be benecial [13,14].

Retention of mechanical properties and oxidation resistance at ele-vated temperatures are important properties for high temperature appli-cations. Purity of sinter additive is an important factor, as surface oxidecomposite. TiO2, SiO2 and Cr2O3 phases were mainly identied on the oxidized surface, which are responsible

Formation of continuous andDensication, characterization and oxidat

T.S.R.Ch. Murthy , J.K. Sonber, C. Subramanian, R.C. HMaterials Group, Bhabha Atomic Research Centre, Mumbai-400 085, India

a b s t r a c ta r t i c l e i n f o

Article history:Received 28 June 2012Accepted 12 October 2012

Keywords:Pre-alloyed (TiCr)B2Hot hardnessFracture toughnessCTEElectrical resistivity

This paper presents the resulof (TiCr)B2 with MoSi2 as sintdensity, hardness, indentationess andelectrical resistivity.in the temperature range ofand oxidized samples were d(TiCr)B2 composite of 4.81 gwith 35 MPa pressure. Hardrange of 35 MPam1/2. Coe

6 1es the high temperaturetemperature applications.

: +91 22 2550 5151.).

rights reserved.n studies of (TiCr)B2+20% MoSi2bli, N. Krishnamurthy, A.K. Suri

f detailed studies carried out on synthesis of starting materials and densicationdditive by hot pressing. Dense compacts were characterized bymeasurement ofacture toughness, exural strength, coefcient of thermal expansion, hot hard-idation studieswere investigatedup to 1000 C and isothermal oxidation kineticsto 950 C. Phase identication and surface morphology analysis of hot pressedusing XRD, SEMEDS and EPMA.

density was obtained with the addition of 20% MoSi2 by hot pressing at 1800 Cs values of composites were close to 27 GPa and fracture toughness was in theient of thermal expansion was measured and found to be in the range of 6.6

etals and Hard Materials

ev ie r .com/ locate / IJRMHMsistance and resistance to chemical attack. Hot pressed CrB2 has beenfound to be oxidation resistant up to 1000 C owing to the formation ofprotective CrBO3 scale [16]. Though somework has been reported earlieron hot pressing of pre-alloyed (TiCr)B2 composite only, detailed studieson the preparation and characterization of (TiCr)B2+20% MoSi2 com-posite have not been reported so far.

This paper reports on the synthesis of starting powders and densica-tion of (TiCr)B2 using MoSi2 as sinter additive. Densied compacts were

evaluated formechanical (hardness, fracture toughness,exural strength,hot hardness) and physical properties (CTE, electrical resistivity). Phaseidentication and microstructural characterizations were carried out byXRD and EPMA. Results of detailed oxidation studies in the temperaturerange of RT to 1000 C, isothermal oxidation at 750, 850 and 950 C andanalysis of fractured and oxidized surfaces by XRD and SEMEDS arepresented.

2. Experimental procedure

2.1. Materials

(TiCr)B2 and MoSi2 powders were used as starting materials in thepresent study. Both powders were prepared in our laboratory by pyrometallurgical methods as described below.

2.1.1. Pre-alloyed titanium and chromium diboride [(TiCr)B2]Synthesis of (TiCr)B2 was carried out by carbothermic reduction of

the respective oxides of titanium and chromium in the presence ofboron carbide as per the following reaction.

2TiO2 B4C Cr2O3 6C2TiCrB2 7CO 1As the reactants consist of four solid substances, homogenization of

the charge mixture is essential for the completion of the reaction. Fine

powders are summarized in Table 1. Data on a typical experiment of(Ti0.85Cr0.15)B2 samples is given in Table 2.

2.1.2. Molybdenum silicide (MoSi2)Molybdenum silicide (MoSi2) was synthesized from its elements

as per the following reaction.

Mo 2SiMoSi2 2Stoichiometric amounts of pure molybdenum (>99.7%, Merck

Germany) and silicon (>99%, Leco industries) were accurately weighedso as to form the compound MoSi2 and thoroughly mixed using plane-tary ball mill. The compacts were heated in a moderate vacuum of0.1 Pa. up to a temperature of 13405 C. After completion of the reac-tion, cooled product of MoSi2 obtained in the form of chunks wascrushed by jaw crusher and ne ground to micron size by vibratorycup grinding mill. After grinding the mean particle size diameter (D50)would be around 1.4 m. Data on a typical experiment is given inTable 2.

D50 (m)

Carbon Assam 99.40 13.9

13T.S.R.C. Murthy et al. / Int. Journal of Refractory Metals and Hard Materials 37 (2013) 1228Carbon,India

Mo Merck,Germany

99.7% purity 325mesh

Silicon Lecoindustries

99.99% purity 325mesh

(Ti0.85Cr0.15)B2

In-house 0.07 0.6 0.5 Ti 60.9%B 26.2%Cr 11.6%

5.0

MoSi2 In-house 0.04 0.3 Fe 0.15% 1.40TiO2 Merck,GermanyCAS no:13463-67-7

99% purity Pb0.005As0.0005Fe0.005

0.80

Cr2O3 Analyticalgrade

>99% purity 0.90

B4C BoroncarbideIndia

19.50 0.5 0.2 Fe~2000 ppmSi~2000 ppm

6.7powders of reactants were accurately weighed and mixed thoroughlyusing a planetary ball mill. The powder mixture was then compactedto obtain green pellets with sufcient strength for handling. The pelletswere loaded into a vacuum induction furnace in a graphite crucible andheated under vacuum. Temperature of the chargewasmeasured using atwo color pyrometer. After completion of the reaction, the furnace wascooled to room temperature in a vacuum and reacted pellets werecrushed and ground to ne size using high energy cup grinding mill.All grinding/mixing equipments are linedwithWC tominimize contam-ination fromgrindingmedia. XRD patterns of the powderswere used forphase identication and chemical analysis for the major constituentsand impurities. Particle size analysis was carried out by laser diffractionmethod (CILAS PSA 1064 L). Details of raw materials and synthesized

Table 1Details of raw materials and synthesized compounds used in the present study.

Material Supplier Carbon(wt.%)

Oxygen(wt.%)

Nitrogen(wt.%)

Metallicimpurities(wt.%)

MedianparticlediameterPowder characteristics of rawmaterials and synthesized powders aregiven in Table 1. Mean particle diameters and its distribution of startingpowdersweremeasured by laser diffractionmethod (CILAS PSA 1064 L)and presented in Fig. 1. Mean particle diameter sizes (D50) of powdersare 5.504, 6.064 and 3.004 mfor (Ti0.85Cr0.15)B2,MoSi2 andmixed pow-ders respectively. Bi-modal distribution and uni-modal distribution pat-tern were observed for (Ti0.85Cr0.15)B2 and MoSi2 powders, whereasmixed powders show the uni-modal distribution pattern with a slightshift towards lower particle size. This indicates that while mixing somemore grinding has taken place. Phase identication of the powders wascarried out by XRD and is presented in Fig. 2. All peaks correspondtoTiB2 and CrB2 (Fig. 2a) and MoSi2 (Fig. 2b) respectively.

2.2. Densication

For hot pressing experiments, weighed quantities of (TiCr)B2 (5,10and 15 wt.% CrB2 equivalent powders) and 20 wt.% MoSi2 powderswere mixed thoroughly using a motorized pestle andmortar in dry con-dition for 1 h. The powderswere thenuniaxially pressed using a graphitedie of 12 mm diameter at 1800 C under pressure (35 MPa) for 1 h in avacuum of 0.01 Pa. The densied pellets were ejected from the die andthe density was measured by Archimedes' principle. Samples werethen cut into different sizes using CNC wire cut EDM (Ultracut S0, Elec-tronicaMachine Tools Ltd). Cut sampleswere hotmounted and polishedwith a diamond slurry of 15, 9, 1 and 0.25 m to obtain mirror nish.Polished samples were used for phase identication, hardness, hot hard-ness, fracture toughness, CTE, electrical resistivity, specic heat, thermal

Table 2Data on a typical experiment on synthesis of (Ti0.85Cr0.15)B2 and MoSi2.

Synthesis of (Ti0.85Cr0.15)B2

Charge components TiO2, B4C, Cr2O3 and carbonCharge weight TiO2 208.03 g, B4C 81.56 g, Cr2O3 48.94 g,

C 51.76 g, total=390.29 gSynthesis conditions 1825 C, 2 h, 0.01 PaProduct weight 211.79 gWeight loss 45.73% (theoretical weight loss 43.08%)Analysis of (TiCr)B2 Ti 60.9%, B 26.2%, Cr 11.6%

Synthesis of MoSi2

Charge composition Mo 63.2%, Si 36.8%Charge weight Mo 125.34, Si 72.99, total=198.33 gSynthesis conditions 1340 C (Till ignition starts), 0.1 PaProduct weight 196.77 gWeight loss 1.56%Analysis of MoSi2 C 0.37%, O 0.3%

B2 D

14 T.S.R.C. Murthy et al. / Int. Journal of Refractory Metals and Hard Materials 37 (2013) 1228diffusivity and exural strength measurement, fractography, and oxida-tion studies.

2.3. Characterization

2.3.1. Microstructure and mechanical propertiesXRD pattern of dense pellets were obtained using Cu K (1.5404 )

radiation and analyzed using the latest available PCPDF database. Phaseanalysis of the polished sample was carried out by EPMA with WDS.Microhardness was obtained with a load of 1.961 N (dwell time 15 s)using a diamond indenter. Indentation fracture toughness (KIc) calcula-tions were based on the crack length measurements of the radial crack

Fig. 1. Particle size distribution plots of starting powders: (Ti0.85Cr0.15)pattern produced by Vickers Hv10 indentations, according to the modelof Anstis et al. [17]. The reported values of both hardness and fracturetoughness are the average data obtained from at least ve indentationtests. Fractured surfaces were obtained under an indentation load of50 kg and were examined in SEM (CAMSCAN MV2300CT) to under-stand the type of fracture, crack propagation and grain coarseningmechanism. Test specimens of composites for 3-point bend tests werecut using EDM and machined into bar shapes of 1.52.025 mmfrom hot pressed disk (50 mm) prepared under similar processing con-ditions. Flexural strength was measured by a 3-point bend test at RT,500 and 900 C as per ASTM C 1161-02C test procedure. Hardness of(Ti0.85Cr0.15)B2+20% MoSi2 composite was measured using diamondindent from room temperature to 1000 C at an interval of 100 C ininert atmosphere.

2.3.2. Thermal propertiesElectrical conductivity from room temperature to 1000 C was

measured by using the conventional 4-probe method. Rectangularcross sectional (dimensions: 1 mm2 mm12 mm) samples wereprepared for conductivity measurement. All sides of the sample werecleaned and polished. Four copper probes were connected to the sam-ple with the help of platinum paste. Two probes were located on thetwo longitudinal sides, which were connected to the programmablecurrent source (KEITHLEY Model 224) and remaining two probeswere connected on the top surface of transverse direction with1 mm apart, which were connected to nanovoltmeter (KEITHLEYModel 2182), which recorded the output voltage when 100 mA currentwas passed through the sample. Electrical conductivity was calculatedusing the following equation.

Conductivity IL=VA 3

where I is current (100 mA), L (1 mm) distance between two probes,which are connected to voltmeter, V is recorded voltage in nanovolt-meter, A is the cross-sectional area of the specimen (mm2).

CTE (coefcient of thermal expansion) was measured up to 900 Cin inert atmosphere using dual push rod dilatometer.

Thermal diffusivity of the samples in the temperature range of roomtemperature to 500 Cwasmeasured using a LASER FLASH thermal dif-fusivitymeasuring system,whichworks on theash or pulse technique.

50:5.504 m; MoSi2 D50:6.064 m; and mixed powder D50:3.004 m.The sample used was a disc of diameter 10 mm and thickness 1 mm.The thermal diffusivity data was corrected for radiation losses and nitepulse width effects. The thermal conductivity () was calculated usingthe denition of thermal diffusivity

kCp

4

where k is the thermal conductivity, is the geometrical density and Cpis the specic heat capacity. The specic heat was determined using DSCby comparisonwith Sapphiremethod. The sampleswere encapsulatedin an aluminum pan and heated in DSC from 30 to 550 C at 5 C/minrate of heating.

Isothermal oxidation studies for all the composites were conductedat 750, 850 and 950 C in air at different time intervals of 0.5, 1, 2, 4, 8,16, 32 and 64 h. For this study, polished samples of size 1055 mmwere loaded into the furnace in an alumina crucible when the furnacetemperature reached the set value. Each sample was carefully weighedbefore and after exposure to determine the weight changes during ox-idation. TGA analysis of the composites was carried out up to 1000 Cin 1 atmosphere oxygen with 10 C/min heating rate (Thermo Sys,Setaram, France) for continuous oxidation studies. Reaction productsformed were identied by XRD. The morphology and nature of oxidelayer were examined by SEM. The cross sections of (Ti0.85Cr0.15)B2+20% MoSi2 composite were studied. Elemental X-ray maps werealso obtained on the cross sections of this composite after isothermaloxidation of 256 h.

15T.S.R.C. Murthy et al. / Int. Journal of Refractory Metals and Hard Materials 37 (2013) 1228(a)3. Results and discussion

3.1. Synthesis of starting powders

3.1.1. Pre-alloyed titanium and chromium diboride [(TiCr)B2]Synthesis of composite boride ((TiCr)B2) would be advantageous

in obtaining a homogeneous product with improved properties. Thiscarbide-oxide reaction is the simplest method for the preparation ofcompound, when the reactants are available in pure and nely dividedform. As the reactants consist of four solid substances, homogenizationof the chargemixture is essential for the completion of the reaction. Theproducts of reaction at ~1200 C were found to be (TiCr)B2 but highlycontaminated with carbon and oxygen, each analyzing about 34%. Asthe temperature of the reaction was increased, the carbon and the oxy-gen contents decreased, similar to synthesis of TiB2 [18] and a pureproduct with carbon and oxygen contents low with ~0.5% each couldbe obtained at a temperature of 1825 C and a nal vacuum of0.001 Pa. A little higher weight loss of 2.65% (actual weight loss:45.73%) was observed compared with theoretical weight loss of43.08%, it may be due to the evaporation of high vapor pressure species

(b)

Fig. 2. XRD pattern of starting powderand moisture. The product of reduction at 1825 C is a loosely sinteredpowder and could be easily crushed to pieces. Further grinding to mi-cron size is carried out in high energy vibratory cup grinding millusing tungsten carbide lined accessories.

3.1.2. MoSi2Molybdenum silicide (MoSi2) was synthesized from its elements

using stoichiometric amounts of puremolybdenum (>99.7%) and silicon(>99%). As the formation of MoSi2 is highly exothermic, abrupt increasein the charge temperature indicates the vigorous reaction. The furnacepower is turned off with the onset of peak temperature. Quantity ofcharge and the spacing between the pellets were adjusted such that theproduct MoSi2 was not overheated to near melting point. A small weightloss of 12%was observed due to evaporation of high vapor pressure spe-cies such as SiO and MoO3.

3.2. Densication

Data on densication of hot pressed samples are presented in Table 3.The density of composites obtained with 5, 10 and 15% CrB2 contained

s a) (Ti0.85Cr0.15)B2 and b) MoSi2.

(TiCr)B2+20% MoSi2 composites was 4.43, 4.67 and 4.81 g/cc respec-tively. Densication data of the TiB2+20% MoSi2+15% CrB2 composite[19] is also presented for comparison.

3.3. Phase identication

Fig. 3 presents the XRD pattern of hot pressed (Ti0.85Cr0.15)B2+20%MoSi2 composite. TiB2 and MoSi2 peaks are mainly identied and CrB2peaks are not seen. SEM images of fractured surfaces of (Ti0.95Cr0.05)B2+20% MoSi2 and (Ti0.90Cr0.10)B2+20% MoSi2 composites arepresented in Fig. 4a and b. Both intergranular and transgranular

Hardness values of all composites are in the range of 24 to 27 GPa. Aver-age indentation fracture toughness of 5, 10 and 15% CrB2 contained(TiCr)B2+20% MoSi2 composites was measured as 2.80.2, 3.50.6and 4.50.5 MPa m1/2 respectively. Hardness of (Ti0.85Cr0.15)B2+20%MoSi2 sample from RT up to 1000 C is presented in Fig. 7. As expected,with increase in temperature, decrease in hardness values was noted.

20 25 30 35 40 45 50 55 60 65 70

``

`

`

`

`

* *

# CrB2

# #

* MoSi2

*

Rel

ativ

e In

tens

ity

2 angle (degrees)

*#

` TiB2

Fig. 3. Phase identication of hot pressed pellet of (Ti0.85Cr0.15)B2+20% MoSi2.

Fig. 4. Fractured surfaces of composites of a) (Ti0.95Cr0.5)B2+20% MoSi2 and

16 T.S.R.C. Murthy et al. / Int. Journal of Refractory Metals and Hard Materials 37 (2013) 1228modes of fractures contribute equally to the failure of these composites.No grain growth is observed in both the composites. No porosity is seenin both the composites Fig. 4a and b, indicates the full densication ofthe composites.

(Ti0.85Cr0.15)B2+20% MoSi2 composite was analyzed by EPMA. Inthis sample, three regions of interest were identied. They are thelight grey matrix, dark grey and black spots (Fig. 5a). To identify theelements on these regions, spectrum was analyzed separately and itwas found that the elements predominantly present in the matrixare Ti, Cr and boron. Lighter regions contain Cr, Ti, Mo and B. Moand Si are the only elements present in the dark patches. Quantitativeanalysis was done on the three identied regions to determine thechemical composition of the elements present, which are shown inTable 4. Elemental mapping of the sample for Ti, Cr, Mo, B and Siand their overlay are shown in Fig. 5bg. Distribution of Ti, Cr and Bin the matrix is shown in Fig. 5b, c and d respectively. These three el-ements appear to be uniformly distributed throughout the matrix.The qualitative analysis of these elements as seen by a variation ofbrightness matches with the composition of the alloy. Fig. 5e presentsthe overlay of Ti+B+Cr in the matrix, which conrms the uniformdistribution of these three elements in the matrix. Fig. 5f presentsthe distribution of silicon indicating that silicon is not uniformly dis-tributed but present as clusters of 12 m size in the matrix. Fig. 5gshows the uniformdistribution ofMo throughout thematrix. This clearlyindicates that MoSi2 has reacted with (TiCr)B2. Mo has diffused and dis-tributed in the matrix, leaving Si undisturbed. The white spots in Fig. 5findicate the location of original MoSi2 particles. The elemental analysisof the polished surface very clearly indicates the presence of Ti, Cr, B andMo uniformly distributed throughout thematrix and silicon as a secondphase precipitate in the matrix.

Line scan across different phases of the sample is shown in Fig. 6.Scan for Si indicates the non continuous presence of Si and localized.At these locations the contents of Ti, B, Cr and also Mo are compara-tively low. Otherwise, these four elements are more or less uniformthroughout the region of line scan. Line scan analysis is in full agree-ment with the results of elemental mapping.

3.4. Mechanical and physical properties

Table 3 presents the hardness and indentation fracture toughnessvalues of the various hot pressed (TiCr)B2+20% MoSi2 composites.

Table 3Density, hardness and fracture toughness of (TiCr)B2+20% MoSi2 composites. Hotpressing conditions: 1800 C, 35 MPa, 1 h.

Sr.no.

Composite Density,g/cc

Vickershardness(GPa)

Microhardness(GPa)

Fracturetoughness(MPam1/2)

1 (Ti0.95Cr0.5)B2+20% MoSi2 4.43 211 251 2.80.22 (Ti0.90Cr0.10)B2+20% MoSi2 4.67 211 241 3.50.63 (Ti0.85Cr0.15)B2+20% MoSi2 4.81 221 271 4.50.54 TiB2+20% MoSi2+15%CrB2

[19]4.64 181 4.10.3b) (Ti0.90Cr0.10)B2+20% MoSi2.

17T.S.R.C. Murthy et al. / Int. Journal of Refractory Metals and Hard Materials 37 (2013) 1228Drastic decrease in the hardness value from 30 GPa to 16 GPa was ob-served from RT to 300 C. Flexural strength was measured at RT, 500and 900 C for (Ti0.85Cr0.15)B2+20% MoSi2 sample and the data is alsopresented in Fig. 7. Three samples were tested at each temperature andthe average values were 320, 295 and 285 MPa respectively. Similar tohot hardness, the decrease in exural strength values with the increasein temperaturewas noticed. However, the decrease in strength is not sig-nicant, indicating the retention of strength at elevated temperatures.

Microstructures of (TiCr)B2+20%MoSi2 composites with indentationedge cracks are shown in Fig. 8a and b. More crack deections wereobserved in (Ti0.95Cr0.05)B2+20% MoSi2 composite, where as both crackdeections and bridging mechanisms were observed in (Ti0.90Cr0.10)B2+20% MoSi2 composites. These mechanisms are responsible for mod-erate improvement of the fracture toughness.

Coefcient of thermal expansion (CTE) and electrical resistivity of(Ti0.85Cr0.15)B2+20% MoSi2 composite is presented in Fig. 9. CTE valuesare found to increase with temperature from 6.67106 at 100 C to7.54106 K1 at 800 C. These values are marginally lower than that

Fig. 5. Back scattered (BSE) image and elemental maof monolithic TiB2 which is 8.1106 K1. Electrical resistivity of thecomposite was measured as 2.12, 2.63 and 3.03 cm at 100, 500 and1000 C respectively. It is observed that coefcient of resistivity is positiveand almost linear. Electrical resistivity of the monolithic TiB2 has beenreported to be in the range of 7 to 15 cm at RT depending on the pu-rity and method of preparation [20,21]. Resistivity of polycrystalline TiB2varies between 10 and 40 cm at room temperature and up to4080 cm at 900 C [21]. High density (4.81 g/cc)and homogeneous

pping of (Ti0.85Cr0.15)B2+20% MoSi2 composite.

Table 4Quantitative analysis of different phases of (Ti0.85Cr0.15)B2+20% MoSi2 composite.

Element Matrix Light patches Dark patches

Titanium 59.74 3.31 0.08Boron 33.17 17.77 0.00Chromium 3.68 54.48 0.82Molybdenum 3.46 26.31 61.38Silicon 0.04 0.03 35.29

of

18 T.S.R.C. Murthy et al. / Int. Journal of Refractory Metals and Hard Materials 37 (2013) 1228solid solution ((TiCr)B2) of our sample could be the reasons for lowerelectrical resistivity value.

Specic heat capacity (Cp), thermal diffusivity and thermal con-ductivity of (Ti0.85Cr0.15)B2+20% MoSi2 composite measured up to600 C are shown in Fig. 10ac. Cp value increases with the increasein temperature. The measured specic heat values were used for thecomputation of thermal conductivity. Thermal conductivity of the

Fig. 6. Line scans across different phasessample was observed to decrease from room temperature to 300 Cand then increase with temperature.

3.5. Oxidation studies

3.5.1. Continuous oxidation (non-isothermal)TGA plot on continuous oxidation of all (TiCr)B2+20%MoSi2 com-

posites in O2 is given in Fig. 11. In the plot, three different stages ofweight gain are observed for all the composites: 1) no change in sam-ple weight up to 300 C, 2) a gradual weight gain between 300 and700 C and 3) a steep weight gain above 700 C. Total specic weightgain is less than 25103 kg/m2 and the rate of weight gain is simi-lar for all the composites. (Ti0.85Cr0.15)B2+20% MoSi2 composite'stotal weight gain(b20103 kg/m2) is lesser compared with theother two composites.

Morphology of the oxidized surface of (TiCr)B2+20% MoSi2 sam-ples after continuous oxidation up to 1000 C (TGA) in oxygen ispresented in Fig. 12af. (Ti0.95Cr0.05)B2+20% MoSi2 oxidized sampleis showing the presence of severe cracks on the surface with thickglossy layer. The number of cracks decreased with an increase inCrB2 content of (Ti0.90Cr0.10)B2+20% MoSi2 composite. Fully protec-tive and crack free glassy layer was seen on the surface of(Ti0.85Cr0.15)B2+20% MoSi2 composite. This indicates the effective-ness of CrB2 content in the composite on the oxidation behavior. Sim-ilar results were observed on the addition of CrB2 to TiB2+MoSi2composites [19].

3.5.2. Isothermal oxidationWeight gain data obtained during isothermal oxidation at 750,

850 and 950 C as a function of time for all the composites arepresented in Fig. 13ac. Continuous weight gain with time is observedin all the samples and at all temperatures. However the rate of weightgain is different for different composites and temperatures. With in-creasing CrB2 content, decrease in weight gain rate is observed. Therecorded values of specic weight gain after oxidation for 64 h at750 C were 4.73, 2.32 and 3.07103 kg/m2 for 5, 10 and 15%CrB2 composites respectively (Fig. 13). The recorded values of specic

(Ti0.85Cr0.15)B2+20% MoSi2 composite.weight gain after oxidation for 64 h at 850 C were 34.62, 26.07 and8.17103 kg/m2 for the same composites (Fig. 13). The recordedvalues of specic weight gain after oxidation for 64 h at 950 Cwere 57.32, 39.58 and 4.58103 kg/m2 for the same composites(Fig. 13). Comparing the weight gain between 750 and 950 C, it is no-ticed that weight gain increases with temperatures for all the compos-ites. But at 950 C a low specic weight gain was observed for 15%CrB2 composites compared to that oxidized at 850 C. The effectivenessof pre-alloyed (TiCr)B2 on oxidation behavior as compared with theseparate addition of TiB2 and CrB2 is brought out below. Specic weight

0 100 200 300 400 500 600 700 800 900 1000

8101214161820222426283032

Temperature, C

Hot

Har

dnes

s, G

Pa

Hot hardnessFlexural Strength

200

220

240

260

280

300

320

340

Flexural strength, MPa

Fig. 7. Hot hardness and exural strength values of (Ti0.85Cr0.15)B2+20% MoSi2composite.

100 150 200 250 300 350 400 450 500 550 600400

450

500

550

600

Spec

ific

heat

Temperature (o C)

0.077

0.078

0.079

0.080

0.081

0.082

al d

iffus

ivity

cm

2 /sec (b)

(a)

(b)

Crack deflections

19T.S.R.C. Murthy et al. / Int. Journal of Refractory Metals and Hard Materials 37 (2013) 1228Crack deflections

Crack bridging gain of TiB2+CrB2+20% MoSi2 composites [19] was higher at all timeintervals compared with the (TiCr)B2+20% MoSi2 samples. From theabove, it can be readily inferred that the composite with pre-alloyed(TiCr)B2 with the addition of 20% MoSi2 has low specic weight gainduring oxidation compared to TiB2 composites that were formed with-out pre-alloyed (TiB2+CrB2).

0 100 200 300 400 500 600 700 800 900 10002.0

2.2

2.4

2.6

2.8

3.0

3.2 Electrical resistivtyCoeffecient of thermal expansion

Elec

tric

al re

sist

ivity

x 1

06 o

hm -

m

6.56.66.76.86.97.07.1

7.2

7.37.4

7.5

7.67.7

7.87.98.0

Co

efficient of Thermal

expan

sion x 10

6 / K

Temperature, (C) Fig. 9. Coefcient of thermal expansion and electrical resistivity data of (Ti0.85Cr0.15)B2+20% MoSi2 composite evaluated in inert atmosphere.

Fig. 8. Crack propagation patterns of a) (Ti0.95Cr0.05)B2+20% MoSi2 b) (Ti0.90Cr0.10)B2+20% MoSi2 composites.650

700

750

800

850

900

cap

acity

J/K

g/K (a)In order to understand the nature of oxidation, the oxidation datawas tted in the general rate equation.

wA

m Kmt 5

where w is the change in weight, A surface area of the sample, toxidation time and Km rate constant. Km and m values at different

120 215 310 408 509 6110.074

0.075

0.076

Ther

m

19202122232425262728293031

Ther

mal

con

duct

ivity

W/m

/K

(c)(c)

Temperature oC

Temperature oC 120 215 310 408 509 611

Fig. 10. a) Specic heat capacity, b) thermal diffusivity and c) thermal conductivitydata of (Ti0.85Cr0.15)B2+20% MoSi2 composite evaluated in inert atmosphere.

(Ti0.85Cr0.15)B2+20% MoSi2 composite, where as it was ~100 m for

20 T.S.R.C. Murthy et al. / Int. Journal of Refractory Metals and Hard Materials 37 (2013) 1228temperatures are presented in Table 5. To understand the kinetics ofoxidation, the data was analyzed using the parabolic law.

wA

2 Kpt 6

Kp is parabolic rate constant. Kp values at different temperaturesare presented in Table 5. The nature of oxidation appears to be completeparabolic (m=2.1 to 2.6) for 5 and10% CrB2 composites and cubic(m>3) for 15% CrB2 composite at 750 C. Parabolic rate constantvalues of 5, 10 and 15% CrB2 composites are measured as 1.25, 0.23,0.531010 Kg2m4 s1 respectively at 750 C. One order higher para-bolic rate constants are measured for the same composites at 850 C inthe range of 3 to 5109Kg m4 s1. The above parabolic rate constantvalues at 750 and 850 C indicate that mechanism of oxidation is diffu-sion controlled. The value of parabolic rate constant is found to decreasewith the increase of CrB2 content from 5 to 15%, indicating the effective-ness of CrB2 addition in improving the oxidation resistance.

XRD patterns of surface layer of all the composites oxidized at 750,850 and 950 C for 64 h are shown in Fig. 14ac. TiO2, Cr2O3 and SiO2phases were identied in all the composites after oxidation at 750 and

100 200 300 400 500 600 700 800 900 10000

5

10

15

20

25

Spec

ific

wei

ght g

ain,

Kg/

m2

x 1

03

Temperature, oC

(Ti0.95Cr0.05)B2+20% MoSi2(Ti0.90Cr0.10)B2+20% MoSi2 (Ti0.85Cr0.15)B2+20% MoSi2

Fig. 11. TGA plot of continuous oxidation of (TiCr)B2+20% MoSi2 composites in O2.850 C. TiO2, CrO3, B2SiO5 phases were identied in all the compositesafter oxidation at 950 C. This indicates interaction of B2O3 and SiO2resulting in the formation of a protective B2SiO5 phase.

SEM images of oxidized surfaces of all composites (5, 10 and 15%CrB2)at 750, 850 and 950 C for 64 h are presented in Figs. 1517 respectively.At 750 C, the start of formation of a liquid phase can be seen on (Fig. 15a)5% CrB2 contained sample. More amount of liquid phase is spreading onthe surface of 10 and 15% CrB2 contained composites (Fig. 15b and c).At 850 C, surface cracks were seen on the 5% CrB2 oxidized sample(Fig. 16a). 10% and 15% CrB2 contained samples were fully covered withdense oxide layer (Fig. 16b and c). At 950 C, more continuous and pro-tective oxide layer was seen with increasing CrB2 content in the sample(Fig. 17ac).

To understand the role of various elements during oxidation of(Ti0.85Cr0.15)B2+20%MoSi2 composite, extended oxidationwas carriedout at 850 C for 256 h. Cross section of the 15% CrB2 contained (TiCr)B2+20% MoSi2 oxidized surface after 256 h was characterized bySEMEDS and is presented in Fig. 18. The thickness of the oxide layeris b10 m and the elemental distribution of this layer for Si, Ti, B, Mo,Cr and O is also presented. The outer most layer of the oxidized surfaceis seen to be that of silicon oxide. Only in the top layer of up to 10 m isthere a deciency of the metallic elements. This layer is dominated bysilicon and oxygen, clearly indicating the migration of silicon towardsTiB2+15% CrB2+MoSi2 composite. From the above discussion, it isclear that pre-alloyed (TiCr)B2+20%MoSi2 composite has better ox-idation resistance compared to TiB2+CrB2+20% MoSi2 compositesprepared from a mixture of individual components. The reason for im-proved oxidation resistance in this (TiCr)B2+20% MoSi2 composite isdue to the migration of silicon towards the outer surface, which formsa protective glassy phase, impeding further oxidation. In the presentstudy, the formation of a continuous and protective layer of TiO2, SiO2and Cr2O3 is noticed. Recent publications have reported improvementin densication, mechanical properties and oxidation resistance by theaddition of silicides (TiSi2, CrSi2) to other refractory metal borides(ZrB2 and HfB2) [2224].

4. Conclusions

1. High dense composites (4.81 g/cc) of pre-alloyed (TiCr)B2+20%MoSi2 were prepared.

2. Moderate hardness in the range of 2427 GPa and fracture toughnessvalues in the range of 35 MPam1/2 were recorded.

3. With increased CrB2 (5 to 15%) content, failure of this compositewas found to change from predominantly transgranular to mixed(trans and intergranular) mode.

4. The nature of oxidation of the composite changed from parabolicto cubic with the increase of CrB2 content from 5 to 15%.

5. Protective oxide layers of TiO2, Cr2O3 and SiO2 were identied onthe surface of the oxidized samples at 750 and 850 C. B2SiO5phase was identied instead of SiO2 when oxidized at 950 C.

6. Formation of a continuous, crack free and protective thick glassyphase was observed with increased CrB2 content in the composite.This composite is found to be a more oxidation resistant materialcompared to the composite without pre-alloyed TiB2+CrB2+20%MoSi2.

7. Silicon is seen to diffuse towards the surface during the high tempera-ture oxidation.

References

[1] Matkovich VI. Boron and refractory borides. Berlin, Heidelberg, New York:Springler-Verlag; 1977.

[2] Subramanian C, Suri AK. Development of boron based neutron absorber materials.Met Mater Process 2004;16(1):39-52.

[3] Basu B, Raju GB, Suri AK. Processing and properties of monolithic TiB2 based ma-terials. Int Mater Rev 2006;51(6):354-74.

[4] Russias J, Cardinal S, Aguni Y, Fantozzi G, Bienvenu K, Fontaine J. Int J Refract MetHard Mater 2005;23:358-62.

[5] Zhang ZH, Shen XB, Wang FC, Lee SK, Fan QB, Cao MS. Low-temperature densi-cation of TiB2 ceramic by the spark plasma sintering process with Ti as a sinteringaid. Scr Mater 2012;66(34):167-70.

[6] Murthy TSRCh, Basu B, Balasubramaniam R, Suri AK, Subramonian C, Fotedar RK.Processing and properties of TiB2 with MoSi2 sinter-additive: a rst report. J AmCeram Soc 2006;89(1):131-8.

[7] Biswas K, Basu B, Suri AK, Chattopadhyay K. Scr Mater 2006;54:1363-8.[8] RajuGB, Basu B.Densication, sintering reactions, and properties of titaniumdiboridethe outer surface. Cross section results of pre-alloyed (TiCr)B2 compos-ite which is compared with the separate addition of TiB2 and CrB2 arebrought out below. The thickness of the oxide layer is ~100 m forTiB2+15%CrB2+MoSi2 composite, which is much higher than that ofthe presently oxidized (Ti0.85Cr0.15)B2+20% MoSi2 composite ofb10 m. Migration of boron towards the outer surface was observedin TiB2+15%CrB2+MoSi2 composite, where a migration of silicon incase of (Ti0.85Cr0.15)B2+20% MoSi2 composite.

The effect of isothermal oxidation (850 C for 64 h) behavior of (TiCr)B2+20% MoSi2 composites is compared with that of the compositeswithout pre-alloyed TiB2+CrB2+20% MoSi2 composites (Table 5).Weight gains of pre-alloyed composites are relatively lower comparedto the admixed composites without pre-alloying [19]. After 256 h of ox-idation, thickness of the oxide layer was found only b10 m forwith titanium disilicide as a sintering aid. J Am Ceram Soc 2007;90(11):3415-23.

21T.S.R.C. Murthy et al. / Int. Journal of Refractory Metals and Hard Materials 37 (2013) 1228[9] Murthy TSRCh, Subramanian C, Fotedar RK, Gonal MR, Sengupta P, Kumar S. Prep-aration and property evaluation of TiB2+TiSi2 composite. Int J Refract Met HardMater 2009;27:629-36.

[10] Murthy TSRCh, Sonber JK, Subramanian C, Fotedar RK, Kumar S, Gonal MR, et al. Anew TiB2+CrSi2 composite densication, characterization and oxidation studies.Int J Refract Met Hard Mater 2010;28:529-40.

[11] Murthy TSRCh, Sonber JK, Subramanian C, Hubli RC, Suri AK. Densication, char-acterization and oxidation studies of TiB2+WSi2 composite. Int J Refract MetHard Mater 2012;33:10-21.

[12] Murthy TSRCh, Sonber JK, Subramanian C, Fotedar RK, Gonal MR, Suri AK. Effect ofCrB2 addition on densication, properties and oxidation resistance of TiB2. Int JRefract Met Hard Mater 2009;27(6):976-84.

[13] Holcombe CE, Dykes NL. Microwave sintering of titanium diboride. J Mater Sci1991;26(14):3730-8.

[14] Pugach EA, Postolova GG, Ivas'kevich YV, Ogorodnik VV. Scale resistance of doubleTiCr boride. Sov J Superhard Mater 1983;5(4):37-41.

[15] Yamada S, Hirao K, Yamauchi Y, Kanzaki S. B4CCrB2 composites with improvedmechanical properties. J Eur Ceram Soc 2003;23:561-5.

[16] Emiliani ML. Characterization and oxidation resistance of hot pressed chromiumdiboride. Mater Sci Eng A 1993;172:111-24.

[17] Anstis GR, Chantikul P, Lawn BR, Marshall DB. A critical evaluation of indentationtechniques for measuring fracture toughness: I, direct crack measurements. J AmCeram Soc 1981;64(9):533-8.[18] Subramanian C, Murthy TSRCh, Suri AK. Synthesis and consolidation of titaniumdiboride. Int J Refract Met Hard Mater 2007;25:345-50.

[19] Murthy TSRCh, Sonber JK, Subramanian C, Hubli RC, Krishnamurthy N, Suri AK.Densication and oxidation behavior of a novel TiB2MoSi2CrB2 composite.Int J Refract Met Hard Mater 2013;36:243-53.

[20] Knigshofer R, Frnsinn S, Steinkellner P, Lengauer W, Haas R, Rabitsch K, et al.Solid-state properties of hot-pressed TiB2 ceramics. Int J Refract Met Hard Mater2005;23(46):350-7.

[21] Mcleod AD, Haggerty JS, Sadoway DR. Electrical resistivities of monocrystallineand polycrystalline TiB2. J Am Ceram Soc 1984;67(11):705-8.

[22] Sonber JK,Murthy TSRCh, Subramanian C, KrishnamurthyN, Hubli RC, Suri AK. Effectof CrSi2 and HfB2 addition on densication and properties of ZrB2. Int J Refract MetHard Mater 2012;31:125-31.

[23] Sonber JK, Murthy TSRCh, Subramanian C, Sunil K, Fotedar RK, Suri AK. Investiga-tions on synthesis of ZrB2 and development of new composites with HfB2 andTiSi2. Int J Refract Metals Hard Mater 2011;29:21-30.

[24] Sonber JK, Murthy TSRCh, Subramanian C, Sunil K, Fotedar RK, Suri AK. Investiga-tions on synthesis of HfB2 and development of a new composite with TiSi2. Int JRefract Met Hard Mater 2010;28:201-10.

(b22 T.S.R.C. Murthy et al. / Int. Journal of Refractory Metals and Hard Materials 37 (2013) 1228(a) (c) (d

(e) (f20 m

20 m

Fig. 12.Morphology of the oxidized surface of a and b) (Ti0.95Cr0.05)B2+20% MoSi2, c and dcontinuous oxidation up to 1000 C, in oxygen (TGA samples).) )

)

20 m

20 m

) (Ti0.90Cr0.10)B2+20% MoSi2 and e and f) (Ti0.85Cr0.15)B2+20% MoSi2 composites after

23T.S.R.C. Murthy et al. / Int. Journal of Refractory Metals and Hard Materials 37 (2013) 1228556065

3

Ti(Cr)B2+20%MoSi2 (5%CrB2)

(a)0 5 10 15 20 25 30 35 40 45 50 55 60 6505

101520253035404550

Time, hr

Spec

ific

wt.

gain

, Kg/

m2

x10 750oC

850oC950oC

05

101520253035404550556065

Ti(Cr)B2+20%MoSi2 (10%CrB2) 750oC850oC950oC

Spec

ific

wt.

gain

, Kg/

m2

x10

3

(b)

05

101520253035404550556065

Ti(Cr)B2+20%MoSi2 (15%CrB2) 750oC850oC950oC

Spec

ific

wt.

gain

, Kg/

m2

x10

3

(c) 0 5 10 15 20 25 30 35 40 45 50 55 60 65

Time, hr

0 5 10 15 20 25 30 35 40 45 50 55 60 65Time, hr

Fig. 13. Specic weight gain vs. time plots of a) (Ti0.95Cr0.05)B2+20% MoSi2, b)(Ti0.90Cr0.10)B2+20% MoSi2 and c) (Ti0.85Cr0.15)B2+20% MoSi2 composites oxidizedat different temperatures.Table 5Specic weight gain and rate constants after 64 h of oxidation at differenttemperatures.

Sample Sp. wt. gain,Kg m2103

Kpa

(Kg2m4 s1)Kmb mc

At 750 C(Ti0.95Cr0.05)B2+20% MoSi2 4.73 1.251010 2.691012 2.687(Ti0.90Cr0.10)B2+20% MoSi2 2.32 2.321011 8.501012 2.156(Ti0.85Cr0.15)B2+20% MoSi2 3.06 5.261011 3.491015 3.590



a Kp parabolic rate constant.b Km rate constant of general rate equation.c m Slope of general rate equation.

(b24 T.S.R.C. Murthy et al. / Int. Journal of Refractory Metals and Hard Materials 37 (2013) 122820 25 30 35 40 45 50 55 60 65 70 75 80



Cr2O3

SiO2

*****

*

**

*

*

Rel

ativ

e In

tens

ity

2 angle (degrees)

* TiO2750oC


(a)

(c)10 15 20 25 30 35 40 45 5

T950oC

*

* *

*

Rel

ativ

e In

ten

sity

2 angleFig. 14. XRD patterns of (TiCr)B2+20% MoSi2 c10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90



Ti(Cr)B2+20%MoSi2 (15%CrB2)850oC

SiO2

Cr2O3*

*

**

*

* * * *

* TiO2

Rel

ativ

e In

tens

ity

2 angle (degrees)

)0 55 60 65 70 75 80 85 90



i(Cr)B2+20%MoSi2 (15%CrB2)

*

**

B2-Si-O5

CrO3*

* * * *

* TiO2

(degrees)omposites at a) 750, b) 850 and c) 950 C.

25T.S.R.C. Murthy et al. / Int. Journal of Refractory Metals and Hard Materials 37 (2013) 1228Fig. 15. SEM images of a) (Ti0.95Cr0.05)B2+20% MoSi2, b) (Ti0.90Cr0.10)B2+20% MoSi2and c) (Ti0.85Cr0.15)B2+20% MoSi2 composites oxidized at 750 C for 64 h.

(26 T.S.R.C. Murthy et al. / Int. Journal of Refractory Metals and Hard Materials 37 (2013) 1228(a)

(c)(e)

Fig. 16.Morphology of the oxidized surfaces of a and b) (Ti0.95Cr0.05)B2+20% MoSi2, c and disothermal oxidation at 850 C for 64 h in air.Whiskers

Whiskers

Whiskers

b)

(d)

Hole

Hole

Hole

(f)

) (Ti0.90Cr0.10)B2+20% MoSi2 and e and f) (Ti0.85Cr0.15)B2+20% MoSi2 composites after

27T.S.R.C. Murthy et al. / Int. Journal of Refractory Metals and Hard Materials 37 (2013) 1228Fig. 17. SEM images of a) (Ti0.95Cr0.05)B2+20% MoSi2, b) (Ti0.90Cr0.10)B2+20% MoSi2and c) (Ti0.85Cr0.15)B2+20% MoSi2 composites oxidized at 950 C for 64 h.

Fig. 18. Cross section of (Ti0.85Cr0.15)B2+20% MoSi2 oxidized surface after 256 h at 850 C.

28 T.S.R.C. Murthy et al. / Int. Journal of Refractory Metals and Hard Materials 37 (2013) 1228

Densification, characterization and oxidation studies of (TiCr)B2+20% MoSi21. Introduction2. Experimental procedure2.1. Materials2.1.1. Pre-alloyed titanium and chromium diboride [(TiCr)B2]2.1.2. Molybdenum silicide (MoSi2)

2.2. Densification2.3. Characterization2.3.1. Microstructure and mechanical properties2.3.2. Thermal properties

3. Results and discussion3.1. Synthesis of starting powders3.1.1. Pre-alloyed titanium and chromium diboride [(TiCr)B2]3.1.2. MoSi2

3.2. Densification3.3. Phase identification3.4. Mechanical and physical properties3.5. Oxidation studies3.5.1. Continuous oxidation (non-isothermal)3.5.2. Isothermal oxidation

4. ConclusionsReferences

Densification, characterization and oxidation studies of (TiCr)B2+20% MoSi2

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