Materials Science & Engineering A 565 (2013) 414–419

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Materials Science & Engineering A

0921-50

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Mechanical properties and in-situ toughening mechanism of pressurelesslydensified ZrB2–TiB2 ceramic composites

Jie Yin a,b, Zhengren Huang a,n, Xuejian Liu a, Yongjie Yan a, Hui Zhang a, Dongliang Jiang a

a State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, Chinab Graduate School of the Chinese Academy of Sciences, Beijing 100039, China

a r t i c l e i n f o

Article history:

Received 8 July 2012

Received in revised form

3 December 2012

Accepted 5 December 2012Available online 11 December 2012

Keywords:

Boride composites

Mechanical characterisation

Pressureless sintering

In-situ toughening

93/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.msea.2012.12.012

esponding author. Tel: þ86 21 52414901; fa

ail address: zhrhuang@mail.sic.ac.cn (Z. Huan

a b s t r a c t

Refractory metal diboride ceramics are promising materials for various high temperature applications.

In this work, systematic studies were conducted on the mechanical properties, including the strength at

elevated temperatures, of in-situ toughened ZrB2–TiB2 ceramic composites that were prepared via

pressureless sintering at 2100 1C for 2 h. Linear shrinkage and open porosity measurements indicated

that the composites were fully densified. Microstructural observations revealed intergranular cracking

behaviour. Thermal residual stress along with elongated SiC platelets contributed to the improvement

in toughness. The toughness was strongly affected by the ZrB2 grain size. The high temperature

degradation of the flexural strength was attributed to the presence of oxide species.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Refractory transition metal diborides, such as zirconiumdiboride (ZrB2) and titanium diboride (TiB2), exhibit propertiesthat make them promising candidates for use in space, industrialand armour applications: high melting points (43000 1C), highstrengths, high hardness and good chemical stability [1–3].

Due to inner strong covalent bonds, fully densified ZrB2–TiB2

ceramic composites are difficult to obtain. Many approaches forenhancing the sintering driving force have been attempted, e.g.,hot pressing (HP) and spark plasma sintering (SPS) [4,5]. Pres-sureless fabrication has several advantages compared with thesemethods, including the ability to prepare near-net-shape partsand to prevent density variations in the final components. Mostimportantly, when pressureless sintering is used, the amount ofsubsequent diamond machining of the sintered parts can besubstantially reduced, thus lowering the production costs.

Because the fracture toughness of borides is relatively low,their sample sizes are confined, and the potential broad applica-tions of boride ceramics are therefore restricted. Compositiondesigning is an important method for improving the performanceof brittle boride ceramic composites. Over the past few decades,efforts have been devoted to finding a suitable reinforcing phase.These methods include whisker-reinforcing, fibre-reinforcingand particulate-reinforcing. Compared with whisker-reinforcedand fibre-reinforced composites, particulate-reinforced ceramiccomposites have significantly fewer problems regarding their

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x: þ86 21 52413903.

g).

processing care and their high temperature degradation [6]. Toimprove the mechanical properties of the composites, the mor-phology of the particles needs to be tailored. The presence of anelongated platelet-like phase contributes to the crack deflectionand to the energy dissipation, leading to higher toughness [7–9].Thus, this platelet toughening technique has attracted attentionin recent years.

In our earlier work, highly toughened ZrB2–SiC–TiB2–B4C(ZSTB) ceramic composites (6.570.5 MPa m1/2) were obtainedvia pressureless sintering [10]. However, the detailed phaseevolution during firing has not been determined. Additionally,systematic studies on the mechanical properties of as-fabricatedZrB2-TiB2 composites are still needed. The purpose of this paper isto study the mechanical properties and, more importantly, the in-situ toughening mechanism of pressurelessly densified ZrB2–TiB2

composites.

2. Experimental procedure

Commercially available ZrB2, SiC, B4C and TiC powders wereused as raw materials. The particle size of the ZrB2 powder wasfirst reduced by attrition milling at 300 rpm for 2 h. Then, thepowder mixtures (listed in Table 1) were ball milled, dry pressedat 60 MPa, isostatically cold pressed at 200 MPa, and finallypressurelessly sintered at 2100 1C. The heating rate was 10 1Cmin�1. The sintering included a vacuum (�6 Pa) firing at tem-peratures below 1600 1C. Details of the powder processing andsintering procedures have been reported elsewhere [10].

Table 1Starting compositions of the powder mixtures (in wt. %) [10].

In-situ carbon ZrB2 SiC B4C TiC Material no.

0 92.0 0 8.0 0 ZB

1.5 84.7 0 10.3 5 ZTB-1.5C

3 77.4 0 12.6 10 ZTB-3C

4.5 70.1 0 14.9 15 ZTB-4.5C

0 81.3 10.7 8.0 0 ZSB

1.5 74.9 9.8 10.3 5 ZSTB-1.5C

3 68.4 9 12.6 10 ZSTB-3C

4.5 62.0 8.1 14.9 15 ZSTB-4.5C

Fig. 1. XRD curves of the ZTB-1.5C powder mixture heated to several key

temperatures ((a) 1600 1C, (b) 2000 1C and (c) 2100 1C), revealing the formation

of a solid solution phase upon heating.

J. Yin et al. / Materials Science & Engineering A 565 (2013) 414–419 415

The phase evolution was analysed using X-ray diffraction(XRD) with a Guinier-Hagg camera (Expectron XDC-1000, JungnerInstrument, Solna, Sweden) and Cu Ka radiation. A 3 mm�4 mm�36 mm specimen was polished to a 1 mm finish usingdiamond abrasives. The three-point flexural strength was testedusing a universal tester (Instron-1195, Instron, Canton, MA) over a30 mm span with a cross-head speed of 0.5 mm min�1. Theelastic modulus was calculated using the bending method basedon the Chinese Standard GB/T 10700-2006. The Vickers hardnesswas measured using the Vickers indentation method (Model 300,Tukon, Canton, MA) with a 5 kg load and a 10 s dwell time. Fivesamples were used for each mechanical test. Micrographs weretaken by a field emission scanning electron microscope (SEM,JSM-6700F, JEOL, Tokyo, Japan) and by a transmission electronmicroscope (JEM-2100F, JEOL, Tokyo, Japan). The oxidation resis-tance of as-sintered bulks was studied by measuring the residualstrength after oxidation at 1400 1C, 1500 1C and 1600 1C with30 min isothermal holds. Firing was conducted in a resistancefurnace (HT 40/17, Nabertherm GmbH, Lilienthal, Germany) at aheating rate of 8 1C min�1. The oxygen content of as-milled ZrB2

powder was determined using the infrared method after fusion inan inert gas atmosphere with a LECO TC-600 analyser.

Fig. 2. (a) Open porosities and (b) linear shrinkages of as-sintered Z(S)TB

composites. The data reflect the high densification level of the composites.

3. Results and discussion

3.1. Phase evolution and densification degree

The as-compacted ZTB-1.5C biscuit was heated to differenttemperatures to evaluate the phase evolution at high temperatures(Fig. 1). TiB2 peaks were detected after heat treatment at 1600 1C [11],as were ZrB2 peaks. This result indicated that TiB2 was formed before1600 1C. The TiB2 peaks disappeared at 2000 1C while the ZrB2 peaksshifted to higher angles and were stable up to 2100 1C. The atomicradius of Ti differs from that of Zr by only 9%; therefore, TiB2 coulddissolve into the ZrB2 lattice to form a continuous solid solution. Thishypothesis was confirmed by other studies [1,12].

The open porosities and linear shrinkages of the sinteredmaterials are shown in Fig. 2. The carbon formed in-situ removedB2O3 gaseous species and contributed to the densification pro-gress [10]. Variations in the linear shrinkage were not observedwhen the in-situ carbon content was less than 3 wt. %. It shouldbe noted that higher shrinkage did not correspond to lower openporosity. This result was presumably due to the evaporation ofgaseous species,

2B2O3þ7C¼B4Cþ6CO(g) (1)

SiO2þ3C¼SiCþ2CO(g) (2)

B2O3 was formed from the starting boride powders, and SiO2

originated from the SiC powder [13]. Evaporation was reduced asthe TiC content increased, which should lead to lower elemental lossand open porosity. Although the in-situ carbon content increased

continuously, the linear shrinkage decreased for Z(S)TB-4.5C. Weassumed that this result was due to the presence of residual carbonbecause Z(S)TB-4.5C had the highest in-situ carbon content,although further studies are needed. Nevertheless, all of the linear

Fig. 3. Mechanical properties of the Z(S)TB composites: (a) flexural strength,

(b) elastic modulus and (c) Vickers hardness as a function of in-situ carbon

content.

J. Yin et al. / Materials Science & Engineering A 565 (2013) 414–419416

shrinkages exceeded 20%, indicating the high densification of theZ(S)TB ceramics. The calculated relative densities were all higherthan 100%, confirming the high densification [10].

3.2. Mechanical properties

The flexural strengths of the Z(S)TB composites are shown inFig. 3a. The flexural strength increased with increasing in-situ

carbon content, and the maximum increase in strength was 38.5%.This result was mainly attributed to the finer microstructure withhigher in-situ carbon content [10]. The highest strength as well thehighest toughness was measured for the ZSTB-4.5C composites.

The elastic moduli of the ZSTB composites are shown in Fig. 3b.The modulus increased with increasing in-situ carbon content upto 1.5 wt% (the highest modulus was 466715 GPa) and thendecreased with higher content. According to the rule of mixtures,the presence of the TiB2 phase would increase the elasticmodulus, while the carbon formed in-situ would decrease themodulus. Earlier researchers measured the elastic modulus usingthe ASTM standard (C1259-01 for impulse excitation of cylind-rical discs) [14]. However, this method is currently unavailabledue to equipment limitations, and further studies are underway.

The hardness of the Z(S)TB composites followed a trend similarto that of the modulus, with a highest value of 17.070.1 GPa(the ZSTB-1.5C material, Fig. 3c). The incorporation of SiC intothe ZrB2-based ceramics led to an increase in hardness becausethe hardness of SiC (Hv �28 GPa) is higher than that of ZrB2

(Hv �23 GPa).

3.3. Microstructural observation

The microstructures of fractured surfaces are shown in Fig. 4.An obvious transition in the fracture mode from intragranular(ZB) to intergranular (ZSTB-4.5C) was observed. The insertedpictures show the propagation of cracks on polished and ther-mally etched surfaces. Crack deflection and branching wereobserved in ZSTB-4.5C. Moreover, the TEM image of ZSTB-4.5Cshowed cracks that propagated along the grain boundary (Fig. 5),indicating weak interphase bonding and intergranular crackingbehaviour.

3.4. In-situ toughening mechanism

The toughness of the ZB and ZSB composites were 2.3 and2.9 MPa m1/2, respectively. The toughness improvement of the ZSceramics was investigated. Micrographs of the polished surfacesof the ZSB and ZSTB-4.5C ceramics are shown in Fig. 6 (some pitsare observed). The difference in the thermal expansion coefficientbetween the matrix and the reinforcing phase would generateresidual stress upon cooling. Because the materials were fullydensified, the average stress in the particulate sp can be calcu-lated using the following formula [15],

sP

Em¼�2ð1�f pÞban

Að3Þ

where fp is the volume fraction of the reinforcing SiC particles (SiCp),b¼ 1þnmð Þ= 1�2np

� �Ep=Em

� �Þ

�, an judges the thermal mismatch

between matrix and SiCp in a certain temperature range, andA¼(1� fp)(bþ2)(1þnm)þ3bfp(1�nm). fp is 0.148 (Fig. 6a). In thiscase, the temperature region was set as 20001C. Hencean¼(6.8�4.5)�10�6

�2000¼4.6�10�3. Thus, the average resi-dual compressive stress (sp) produced by the SiCp is 235 MPa. Thetoughness increase due to thermal residual stress is revealed by thedecrease in the stress intensity factor KI [15],

DK I ¼ 2sP

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2 l�dð Þ

p

rð4Þ

where l is the mean distance between adjacent SiCp and d denotesthe average particle size of the SiCp. The l and d values in ZSB are12.4 mm and 7.2 mm, respectively. Thus, according to these calcula-tions, the toughness increased to 0.8 MPa m1/2. This result is close tothe experimental value of 0.6 MPa m1/2.

The toughness of the ZTB-4.5C and ZSTB-4.5C composites wascompared using the same method. According to Fig. 6b, fp is

Fig. 4. Fractured surfaces of the (a) ZB and (b) ZSTB-4.5C materials. The inserted

pictures show the crack propagation along polished and thermally etched surfaces.

Fig. 5. Transmission electron micrographs of the ZSTB-4.5C composite showing a

crack that propagated along the interphase.

Fig. 6. Typical scanning electron microscopy images of the (a) ZSB and (b) ZSTB-

4.5C [10] composites that were used for the increase in toughness calculation.

Fig. 7. Relation between the ZrB2 grain size and the fracture toughness of the

composites, indicating that the fracture toughness increased as the ZrB2 grain size

decreased.

J. Yin et al. / Materials Science & Engineering A 565 (2013) 414–419 417

0.139, and the l and d values for the SiCp in the ZSTB-4.5Cmaterial are 10.0 mm and 4.6 mm, respectively. Therefore, l�d is5.4 mm, and the theoretical toughness increase should be

J. Yin et al. / Materials Science & Engineering A 565 (2013) 414–419418

approximately 0.8 MPa m1/2. However, the toughness increase isactually 2.4 MPa m1/2 (4.1 MPa m1/2 for ZTB-4.5C and 6.5 MPam1/2 for ZSTB-4.5C), indicating that other toughening mechanismsmust be involved. The presence of a fine and elongated particulatephase in the matrix has been proven to deflect cracks duringpropagation and to release fracture energy, leading to improvedtoughness [16]. In our study, the aspect ratio of the SiCp increasedfrom 3 (ZSB) to 4 (ZSTB-4.5C), while the average size of the SiCp

decreased by 36.1%. Nonetheless, more distinct differences amongthe ZrB2 grain sizes were found. The correlation between the ZrB2

Fig. 8. Residual flexural strength of the ZSTB-4.5C composites after oxidation at

elevated temperatures (1400 1C, 1500 1C and 1600 1C) for 30 min.

Fig. 9. Transmission electron micrographs of the ZSTB-4.5C composite: (a) demonstra

analysis of the black matrix phase in (a).

grain size and toughness is plotted in Fig. 7. The toughnessincreased as the ZrB2 grain size decreased. Due to the presenceof SiC platelets, the grain growth of ZrB2 was curbed in the ZSceramics, and the grain size of ZrB2 in the ZS(T)B composites wastherefore much smaller than that in the Z(T)B composites. BothZrB2 grain sizes in ZTB-4.5C and ZSTB-4.5C were o10 mm,significantly different than those in the ZB and ZSB composites.The most likely reasons for the improvement in toughness withdecreasing ZrB2 grain size are as follows: First, based on the Irwinequation [17],

K IC ¼ Ysffiffiffiap

ð5Þ

where Y is a geometric constant and a is the critical crack size, thestrength was strongly influenced by the grain size (d-1/2, Hall-Petchrelationship). Because no apparent defects were observed in thebulk composites, the critical crack size was assumed to have thesame order of magnitude as the grain size (d). Thus, the fracturetoughness increased with decreasing ZrB2 grain size. Our result wasinconsistent with previous reports [18]. Nevertheless, such a strongdependence of the toughness on the grain size was observed inother ceramic systems [19,20]]. Second, the ZrB2 lattice contractedas the smaller Ti atoms dissolved to form a solid solution. Thus, as acrack propagated, strain was relieved, the crack tip blunted, and thestress intensity factor decreased. The ZSTB-4.5C composite had thehighest amount of dissolved TiB2 in the ZrB2 lattice (�27.5 vol%,Table 1) and therefore had the highest toughness.

3.5. Oxidation resistance characterisation

The residual strength as a function of temperature is shown inFig. 8. Generally, the strength decreased with increasing temperature.

tes the presence of a glassy phase at the grain boundary while (b) shows the EDS

J. Yin et al. / Materials Science & Engineering A 565 (2013) 414–419 419

After oxidation at 1400 1C and 1500 1C for 30 min, the strength was73% and 68% of the initial value, respectively. The strength retentionwas only 48% when the temperature was further increased to1600 1C. This degradation in strength at elevated temperatures wasattributed to the following: First, the presence of a grain boundaryamorphous phase would induce grain boundary sliding at hightemperatures [21]. For as-milled and batched ZrB2 powder, theoxygen content was up to 2.2 wt% in the form of several oxideimpurities (derived from the commercial ZrB2 powder [22]). The TEMimage (Fig. 9a) showed the presence of a glassy phase at tripleboundary junctions. Grain sliding would generate cavities anddecrease the strength. Second, the EDS result (Fig. 9b) verified thatO was dissolved into the matrix lattice. A major proportion of O wasin the form of zirconia, and m-ZrO2 would transform into t-ZrO2 asthe temperature increased, leading to a volume shrinkage of 7%.Therefore, cavities might be formed and induce grain sliding at hightemperatures. Detailed studies are currently underway.

4. Conclusion

The mechanical properties of pressurelessly sintered ZrB2–TiB2

ceramics were studied. High linear shrinkage (420.2%) and lowopen porosity (o0.3%) were observed in the composites. TheZSTB-1.5C material had the highest elastic modulus and hardnessvalues of 466715 GPa and 17.070.1 GPa, respectively, while theflexural strength increased with increasing in-situ carbon content.From the SEM and TEM observations, the crack was shown topropagate along the grain boundary, demonstrating weak inter-phase bonding. The in-situ toughening mechanism was investi-gated, and the ZrB2 grain size strongly influenced the fracturetoughness. After oxidation at 1500 1C for 30 min, the flexuralstrength was 68% of the room temperature value.

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