8
Mechanical Properties of Zirconium-Diboride Based UHTCs

Eric W. Neuman^a and Greg E. Hilmas

Materials Science and Engineering Department, Missouri University of Science and Technology, Rolla, MO, USA

8.1 Introduction

This chapter focuses on the mechanical properties of zirconium diboride (ZrB₂)-based ultra-high temperature ceramics (UHTCs), including elastic modulus, strength, and fracture toughness at ambient and elevated temperatures. The materials covered include ZrB₂ ceramics with additions of various sintering aids, silicon carbide, and transition metal silicides. Reporting of carbide, nitride, and oxide additions is limited to instances where materials were added in small quantities or as sintering aids.

To produce structures using UHTCs, designers need to have a robust set of mechanical property data detailing their elastic properties, fracture toughness, and strength. These data will need to include microstructural relations, as well as specimen processing and testing procedures. Currently, much of the published mechanical property data are not accompanied by microstructural analysis. Further, a wide variety of testing methods are utilized for measuring the mechanical properties. Lack of microstructural characterization confounds the analysis, which is directly related to the microstructural features. Further, the use of various testing standards and specimen preparation methods make comparisons between studies difficult. Microstructural information is included in the proceeding discussion, when provided. Differences in testing methods will be noted, but only minimally discussed.

The chapter also discusses improvements in mechanical properties that have been obtained using modern powders and processing techniques compared to historic studies from the 1960s and 1970s. The main contributions to the improvement in properties of the modern materials are the use of powder with higher purity and finer particle sizes, along with the use of sintering aids and isothermal reaction holds to remove impurities, such as surface oxides, from the starting powders.

8.2 Room Temperature Mechanical Properties

Zirconium diboride (ZrB₂), a transition metal boride compound, is part of a class of materials known as UHTCs. This family of compounds is characterized by melting points in excess of 3000°C [1]. These ceramics are candidates for applications including molten metal crucibles [2, 3], furnace electrodes, cutting tools [4, 5], control rods for fission reactors, and wing leading edges on future hypersonic aerospace vehicles. Additionally, particulate-reinforced ZrB₂ matrix composites, especially those with silicon carbide (SiC) additives, have displayed enhanced properties. ZrB₂–SiC composites exhibit room temperature strengths in excess of 1000 MPa [6–8], fracture toughness values as high as 5.5 MPa · m^½ [6, 7, 9], and hardness values exceeding 22 GPa [6, 8, 10]. These properties are comparable to other commonly used structural ceramics (Al₂O₃, ZrO₂, Si₃N₄, etc.). In addition, ZrB₂ offers chemical stability and increased refractoriness. The following discussion highlights the room temperature elastic modulus, strength, and fracture toughness of ZrB₂-based UHTCs.

8.2.1 ZrB₂

Elastic modulus values reported for selected ZrB₂ ceramics with and without sintering additives are summarized in Table 8.1. Elastic modulus ranges from approximately 350 to approximately 530 GPa depending on porosity and additives (Fig. 8.1). Fitting the data to the Einstein (linear) [42], Spriggs [43], and Nielson [40, 41] models (Table 8.2) gives a value for fully dense ZrB₂ of 511 GPa compared to measured values of 490–500 GPa in previous studies [38, 44]. However, the materials in the historic studies were not fully dense, and fitting the historic data to similar models predicts an elastic modulus of 519 GPa for dense ZrB₂. Zhang et al. [45] calculated the elastic modulus of ZrB₂ from first principles to be 520 GPa, in good agreement with the fitted data. Recent work by Okamoto et al. calculated the polycrystalline elastic modulus of ZrB₂ from single crystal measurements to be 525 GPa [46]. The modulus of 99.8%-dense ZrB₂ was measured to be 489 GPa [26]. However, additions of AlN, Si₃N₄, B₄C, and C, as well as impurities picked up during powder milling (i.e., WC, ZrO₂), affect the elastic modulus of ZrB₂ ceramics. Small additions of B₄C and/or C increase the elastic modulus of ZrB₂ [20, 22, 29]. Additions of AlN and Si₃N₄ lower the elastic modulus [11, 32]. Additions affect the elastic modulus primarily through interactions with impurities on the surface of the starting powders. Additives such as C and B₄C remove low modulus surface oxides, which increases modulus, whereas additives such as AlN (308 GPa) [47] and Si₃N₄ (310 GPa) promote formation of low modulus grain boundary phases, which reduces modulus [44]. The elastic modulus of 99.4% dense ZrB₂ with 0.5 wt% C added as a sintering aid was 524 GPa [29]. This material was nearly phase pure (residual C not observed) and the modulus matched Okamoto's predicted polycrystalline value of 525 GPa. The reported modulus values for ZrB₂ are typically impacted porosity and/or impurities, but the intrinsic elastic modulus seems to be approximately 525 GPa.

Table 8.1. Elastic modulus, Vickers hardness, fracture toughness by direct crack method, and four-point flexure strength of ZrB₂ ceramics with and without sintering additives

Composition	Relative density	Grain size	Elastic modulus	Hardness	Fracture toughness	Flexure strength	References
(vol%)	(%)	(µm)	(GPa)	(GPa)	(MPa · m½)	(MPa)
ZrB2	87	10	346 ± 4	8.7 ± 0.4	2.4 ± 0.2a	351 ± 31	[11–14]
ZrB2	90	—	—	16.1 ± 1.1	1.9 ± 0.4	325 ± 35b	[15]
ZrB2	90.4	6.1	417	—	4.8 ± 0.4	457 ± 58	[16–18]
ZrB2	95.8	10	—	16.5 ± 0.9	3.6 ± 0.3	450 ± 40b	[19]
ZrB2	97	8.1	479 ± 8	16.7 ± 0.6	2.8 ± 0.1	452 ± 27	[20]
ZrB2	97.2	5.4 ± 2.8	498	—	—	491 ± 22	[21]
ZrB2	98	9.1	454	14.5 ± 2.6	—	444 ± 30	[22, 23]
ZrB2	>98	—	—	14.7 ± 0.8	—	300 ± 40	[24]
ZrB2	~99	20	491 ± 34	—	—	326 ± 46	[25]
ZrB2	99.8	~6	489	23 ± 0.9	3.5 ± 0.3c	565 ± 53	[26–28]
ZrB2 + 0.5 wt% C	99.4	19 ± 13	524 ± 17	14.3 ± 0.7	2.9 ± 0.2a	381 ± 41	[29, 30]
ZrB2 + 4 wt% B4C	100	8	530	18	3.1	473	[22]
ZrB2 + 4 wt% Ni	98	5–15	496	14.4 ± 0.8	3.4 ± 0.4a	371 ± 24	[12, 31]
ZrB2 + 4.6 AlN	~92	—	407 ± 5	9.4 ± 0.5	3.1 ± 0.1a	580 ± 80	[32]
ZrB2 + 5 Si3N4	98	3	419 ± 5	13.4 ± 0.6	3.7 ± 0.1a	600 ± 90	[11, 33–35]
ZrB2 + 3 YAG	95	7.5	—	—	5.4 ± 0.2d	629 ± 31b	[36]
ZrB2 + 2 wt% B4C + 1 wt% C	100	4.1	507	19.6 ± 0.4	3.5	575 ± 29	[22, 37]
ZrB2 + 2 wt% B4C + 1 wt% C	100	2.5	509 ± 11	19.7 ± 0.6	3.0 ± 0.1	547 ± 35	[20]

^aChevron notched beam.

^bThree-point flexure.

^cIndentation strength in bending.

^dSingle-edge notched beam.

Figure 8.1. Room-temperature elastic modulus as a function of porosity for ZrB₂ (left) with and without sintering aids [11–14, 16, 17, 20–23, 25–29, 31–35, 37–39]. Line represents fitted relationship of elastic modulus to porosity according to Nielsen's relationship [40, 41].

Table 8.2. Summary of various fitted models of elastic modulus as a function of porosity for ZrB₂

Equation	E0	b	R2
E = E0(1 − bP)	511 ± 10	2.0 ± 0.3	0.6416
E = E0e− bP	512 ± 11	2.4 ± 0.4	0.6614
	511 ± 11	0.8 ± 0.2	0.6665

Room temperature flexure strengths for ZrB₂ ceramics with and without sintering aids are also given in Table 8.1. The strength ranges from 250 to 630 MPa depending on grain size and additives (Fig. 8.2). In general, the strength of ZrB₂ follows an inverse square root relation with grain size (GS^−½), as expected for ceramics free of other larger flaws. The line in Figure 8.2 highlights the GS^−½ relationship, but is not intended as a fit of the data. This relation indicates that flaw-free ZrB₂ with a fine grain size has improved strengths.

Figure 8.2. Room-temperature flexure strength as a function of grain size for ZrB₂ (left) with and without sintering additives [11–14, 16–29, 31, 33, 34, 36, 37, 48]. Line is not fitted to data, and is meant to guide the eye.

The fracture toughness of ZrB₂ is generally in the range of 3.0–4.5 MPa · m^½, with most values being reported near approximately 3.5 MPa · m^½. Unfortunately, the effects of porosity or grain size on fracture toughness have not been investigated. Also, the possibility of R-curve behavior has not been investigated, even though the CTE mismatch between the a-axis (6.9 × 10⁻⁶ K⁻¹) and c-axis (6.7 × 10⁻⁶ K⁻¹) [49] of the hexagonal unit cell leads to residual stresses that could induce R-curve behavior.

8.2.2 ZrB₂ with SiC Additions

8.2.2.1 Size Effect of SiC Addition.

The four-point flexure strength of ZrB₂–SiC ceramics without additives is shown in Table 8.3. Although ZrB₂ grain size is a factor, the room temperature strength of ZrB₂–SiC is controlled by the size of the dispersed SiC particles or clusters [7–9]. Rezaie et al. [7] and Watts et al. [8] used linear elastic fracture mechanics to show that critical flaw sizes correlated to the SiC cluster size in ZrB₂–30 vol% SiC ceramics. This conclusion is consistent with the CTE mismatch between ZrB₂ and SiC (for α-SiC: α_a-axis = 4.3 × 10⁻⁶ K⁻¹, α_c-axis = 4.7 × 10⁻⁶ K⁻¹) [55]. Watts et al. [56] used neutron diffraction to measure the thermal residual stresses present in ZrB₂–30 vol% SiC. They found that the thermal residual stresses begin to accumulate at approximately 1400°C, resulting in residual tensile stresses in the ZrB₂ and compressive stresses in the SiC at room temperature. Assuming constant strain across the interface between SiC and ZrB₂, the maximum tensile stress in the ZrB₂ was determined to be approximately 1000 MPa [8]. Watts predicted a critical grain size range for microcracking between 6.5 and 13.8 μm. This matches well with the critical SiC particle size of approximately 11.5 μm observed by Watts experimentally (estimated by fitting the major axis of the clusters to ellipses, as discussed later).

Table 8.3. Elastic modulus, Vickers hardness, fracture toughness by direct crack method, and four-point flexure strength of ZrB₂–SiC ceramics

Composition	Relative density	ZrB2 Grain size	SiC Grain size	Elastic modulus	Hardness	Fracture toughness	Flexure strength	References
(vol%)	(%)	(µm)	(µm)	(GPa)	(GPa)	(MPa · m½)	(MPa)
ZrB2 + 10SiC	—	3	—	507 ± 4	—	4.8 ± 0.3a	835 ± 35	[13]
ZrB2 + 10SiC	93.2	~3	—	450	24 ± 0.9	4.1 ± 0.3b	713 ± 48	[26, 28]
ZrB2 + 10SiC	97.1	2.2	~0.2	—	—	5.7 ± 0.2c	720 ± 55d	[50]
ZrB2 + 10SiC	97.4	4.5 ± 1.6	0.8 ± 0.4	—	—	—	524 ± 63	[16, 18]
ZrB2 + 10SiC	99.8	4.3 ± 1.4	—	500 ± 16	18 ± 0.9	3.8 ± 0.3	393 ± 114d	[51]
ZrB2 + 15SiC	96.5	4.4 ± 1.7	0.9 ± 0.5	—	—	—	714 ± 59	[16, 18]
ZrB2 + 15SiC	99	2	—	480 ± 4	17.7 ± 0.4	4.1 ± 0.1a	795 ± 105	[52]
ZrB2 + 20SiC	—	1.8	~0.2	—	—	6.8 ± 0.1c	1009 ± 43d	[50]
ZrB2 + 20SiC	97.3	4.2 ± 1.9	0.9 ± 0.5	—	—	—	608 ± 93	[16, 18]
ZrB2 + 20SiC	99.7	~3	—	466	24 ± 2.8	4.4 ± 0.2b	1003 ± 94d	[26, 28]
ZrB2 + 20SiC	99.7	4.0 ± 1.1	—	506	22.1 ± 0.1	4.2 ± 0.8	487 ± 68d	[51]
ZrB2 + 20SiC	5.62 g/cm3	4.1 ± 0.9	—	505 ± 3	21.3 ± 0.7	3.9 ± 0.3	937 ± 84d	[53]
ZrB2 + 30SiC	—	3	~0.2	—	—	5.9 ± 0.2c	860 ± 70d	[50]
ZrB2 + 30SiC	97.2	2.2 ± 1.2	1.2 ± 0.6	516 ± 3	20 ± 2.0	5.5 ± 0.3b	1063 ± 91	[7]
ZrB2 + 30SiC	97.5	3.9 ± 0.9	—	487 ± 12	24.4 ± 0.6	4.4 ± 0.5	425 ± 35d	[51]
ZrB2 + 30SiC	99.8	1.5 ± 1.2	—	510	27.0 ± 2.2	2.1 ± 0.1	800 ± 115	[54]
ZrB2 + 30SiC	99.4	~3	—	—	24.0 ± 0.7	5.3 ± 0.5b	1089 ± 152	[26, 28]
ZrB2 + 30SiC	99.8	1.2 ± 0.4	1.0 ± 0.4	520 ± 7	20.7 ± 1.0	4.6 ± 0.1b	909 ± 136	[9]
ZrB2 + 30SiC	>99	10.6	1.6	541 ± 22	21.4 ± 0.6	—	1150 ± 115	[8]

^aChevron notched beam.

^bIndentation strength in bending.

^cSingle-edge notched beam.

^dThree-point flexure.

Microstructure–mechanical property relationships for ZrB₂-based ceramics are complex. Because the size of the particulate-reinforcing phase controls the strength, a uniform dispersion of SiC is required to maximize strength, even for compositions and starting particle sizes that should be below the percolation threshold for the system. The method of estimating grain size from image analysis data also affects the interpretation of microstructure–property relations. Figure 8.3 shows the flexure strength of ZrB₂–30 vol% SiC as a function of cluster size estimated using equivalent area diameter, one of the most used methods for reporting grain size. This method assumes a spherical particle, and underestimates the critical grain size for microcracking as approximately 3 μm (Fig. 8.3). Figure 8.3 also shows flexure strength as a function of the maximum measured equivalent area diameter (EAD_max). Since failure occurs at the largest flaw, a measurement that reports the largest flaw size is more useful, but still may not accurately describe the behavior of the material. From Figure 8.3, below the critical grain size determined by EAD_max, the strength shows a linear trend with grain size, instead of the inverse square root relationship predicted by Griffith analysis.

Figure 8.3. Room-temperature flexure strength as a function of SiC cluster size (equivalent area diameter) for ZrB₂–30 vol% SiC ceramics produced by hot pressing [7–9].

Figure 8.4 shows flexure strength, elastic modulus, and hardness as a function of the SiC cluster size that was estimated using the maximum measured major axis from ellipses (ME_max) fitted to the SiC clusters. All three properties experience a sudden, discontinuous decrease at approximately 11.5 μm. Below the critical grain size, the strength follows an inverse square root relation with cluster size. Fitting ellipses to the clusters is preferred because it more closely resembles the morphology of the clusters. Taking this analysis even further, measuring the maximum Feret's diameter of the cluster, should more accurately correlate to the flaw size by freeing the analysis from assumptions of circularity.

Figure 8.4. Room-temperature flexure strength, elastic modulus, and Vickers hardness as a function of maximum SiC cluster size (major axis of ellipse) for ZrB₂–30 vol% SiC ceramics prepared by hot pressing. The dashed line indicates the microcracking threshold that occurs at an SiC cluster size of approximately 11.5 μm [8].

8.2.2.2 Effect of SiC Concentration.

The effect of SiC concentration on the strength of ZrB₂–SiC ceramics is shown in Figure 8.5. In general, the strength (Table 8.3) increases with SiC content. From Chamberlain et al., the strength of ZrB₂ increases from 560 to 1090 MPa with addition of up to 30 vol% SiC [26]. However, the strength is still controlled by the size of the SiC inclusions as discussed previously. Chamberlain was able to achieve high strengths through dispersion of the SiC particulate during processing. When percolation clusters begin to form, or the size of the SiC particles is large, strengths will decrease. Liu et al. produced ZrB₂–SiC ceramics using nanometer-sized SiC powder [50]. They produced strengths comparable to Chamberlain, except at the level of 30 vol% additions, where SiC began to form large percolation clusters, resulting in a material with lower strength than expected based solely on the submicron SiC particle size. Zhang et al. produced pressurelessly sintered ZrB₂–SiC that resulted in large asymmetric SiC grains from the long isothermal holding times (3 h) required for densification [57]. Even though Zhang achieved good dispersion of the SiC particles, the resulting SiC grain size (up to 13 μm long) was larger than the critical grain size for microcracking of approximately 11.5 μm, thus resulting in reduced strengths (490 MPa).

Figure 8.5. Room-temperature flexure strength [16, 22, 26, 37, 50, 57] and fracture toughness [16, 22, 26, 37, 50, 57] as a function of SiC concentration for ZrB₂–SiC ceramics produced by hot pressing and pressureless sintering.

The fracture toughness of ZrB₂–SiC ceramics as a function of SiC additions is also shown in Figure 8.5. In general, fracture toughness (Table 8.3) increases with increasing SiC additions. Chamberlain reported a fracture toughness of 3.5 MPa · m^½ for pure ZrB₂, increasing to 5.3 MPa · m^½ for ZrB2–30 vol% SiC. Liu's work [50] showed that toughness followed a similar trend to strength for nanosized SiC, increasing from 3.1 MPa · m^½ for ZrB₂ to 6.8 MPa · m^½ for 20 vol% SiC then decreasing to 5.9 MPa · m^½ for 30 vol% SiC (along with observed formation of percolation clusters). SiC additions increase the fracture toughness of ZrB₂ by increasing the tortuosity of the crack path. Figure 8.6 shows a polished and etched specimen of ZrB₂–SiC exhibiting crack bridging and crack deflection. The ZrB₂ grains typically fail by transgranular fracture, while cracks deflect at or near ZrB₂–SiC interfaces. This behavior is consistent with residual stresses resulting from CTE mismatch between SiC and ZrB₂. Further, R-curve behavior has been observed in ZrB₂–SiC, as expected based on the CTE mismatch between the phases. Kurihara et al. [59] reported falling R-curve behavior for ZrB₂–10 vol% SiC and Bird et al. [60] reported rising R-curve behavior for ZrB₂–20 vol% SiC. Further increases in fracture toughness will likely require second-phase additions with higher aspect ratios, such as whiskers, rods, or platelets (discussed later), or the fabrication of laminate-type architectures [61].

Figure 8.6. Thermally etched cross section of ZrB₂–30 vol% SiC. The image shows the crack path from a Vickers indent with arrows indicating predominantly transgranular fracture for the ZrB₂ grains and crack deflection near the ZrB₂–SiC interfaces.

Reproduced from [58].

The elastic modulus of ZrB₂ with SiC (475 GPa) [62] additions follows a simple linear volumetric rule of mixtures trend (Fig. 8.7). Small variations in elastic modulus from the expected values may be attributed to the impact of processing impurities on ZrB₂–SiC, such as WC from milling media or the effect of thermal residual stresses.

Figure 8.7. Elastic modulus as a function of additive content for selected hot-pressed ZrB₂-based composites with SiC [16, 18, 26, 28, 52, 54, 63] , MoSi₂ [14, 64–67] , and ZrSi₂ [17] additives. Values have been corrected for porosity using a linear relationship and b = 2.0.

8.2.2.3 Effect of Additional Phases.

The effect of various additions to ZrB₂–SiC is summarized in Table 8.4. Additions are typically introduced as sintering aids that may be designed to remove surface oxide impurities, as in the case of C and B₄C, or interacting with surface oxides to form softer grain boundary phases, as in the case of Si₃N₄. ZrB₂–SiC with other additions has similar room temperature mechanical properties to ZrB₂–SiC without additions. Nitride additions typically increase toughness by formation of a weaker grain boundary phase that increases crack bridging and deflection. Han [70] and Wang [71] observed an increased toughness for ZrB₂–20 vol% SiC (from ~4.2 to ~5.5 MPa · m^½) with 5–10 vol% additions of aluminum nitride, while retaining a room temperature flexure strength of 830 MPa. This increase was likely due to improved densification, which resulted in a finer grain size, and the presence of weaker grain boundary phases (BN and Al₂O₃) that enhanced crack deflection. Small additions of oxides (such as YAG [11] and La₂O₃ [19]) follow a similar trend to the nitride additions with minimal improvements in toughness. This shows that small additions of nitrides and oxides can be added to ZrB₂–SiC to provide additional means of controlling densification and microstructure without adversely affecting room temperature mechanical properties.

Table 8.4. Elastic modulus, Vickers hardness, fracture toughness by direct crack method, and four-point flexure strength of ZrB₂–SiC ceramics with various additives

Composition	Relative density	ZrB2 Grain size	SiC Grain size	Elastic modulus	Hardness	Fracture toughness	Flexure strength	References
(vol%)	(%)	(µm)	(µm)	(GPa)	(GPa)	(MPa · m½)	(MPa)
ZrB2 + 20SiC + 1 wt% B	>99.0	7.3 ± 0.8	3.3 ± 0.5	—	16.0 ± 0.4	—	519 ± 31a	[68]
ZrB2 + 27SiC + 1B4C	100	2	1	508 ± 6	22.6 ± 0.9	3.5	720 ± 140	[63]
ZrB2 + 30SiC + 2 wt% B4Cb	100	1.9 ± 0.9	1.2 ± 0.5	513 ± 24	20.2 ± 0.5	4.9 ± 0.4c	682 ± 98	[69]
ZrB2 + 20SiC + 3wt%C + 0.5wt%B4C	99	~10	~8	374 ± 25	14.7 ± 0.2	5.5 ± 0.5d	361 ± 44a	[39]
ZrB2 + 30SiC + 4 wt% B4Cb + 5 wt% Ce	99	2.8 ± 0.2	2 ± 0.8	511 ± 7	—	3.8 ± 0.2f	604 ± 69	[57]
ZrB2 + 20SiC + 2 wt% La2O3	99.6	3.5 ± 0.4	—	—	19.3 ± 0.6	5.2 ± 0.5	600 ± 70a	[19]
ZrB2 + 20SiC + 4Si3N4	98	2.4 ± 0.1	—	—	14.6 ± 0.3	—	730 ± 100a	[11]
ZrB2 + 20SiC + 5AlN	100	3	—	—	19.4 ± 0.6	5.4 ± 0.3d	835 ± 26a	[70]
ZrB2 + 20SiC + 10AlN	100	2.5	—	—	—	5.6 ± 0.5d	831 ± 12a	[71]
ZrB2 + 15SiC + 4.5ZrN	99	—	—	467 ± 4	15.6 ± 0.3	5.0 ± 0.1c	635 ± 60	[72]
ZrB2 + 20SiC + 6ZrC	99.1	~4	—	—	19.0 ± 0.5	6.5 ± 0.4d	622 ± 64a	[73]
ZrB2 + 21SiC + 5ZrC	97.3	<2	—	—	17.2 ± 0.8	5.2 ± 0.4	747 ± 101	[74]
ZrB2 + 13SiC + 15ZrC	>99	—	—	—	18.2	4.3	512 ± 50a	[75]
ZrB2 + 20SiC + 10ZrC	99.8	2.7 ± 0.5	1.6 ± 0.2	—	18.1 ± 0.6	5.3	662 ± 64a	[76]
ZrB2 + 10SiC + 30ZrC	99	2	—	474	18.8 ± 0.5	3.5 ± 0.2	723 ± 136	[67]
ZrB2 + 20SiC + 5VC	>99	1.9 ± 0.3	1.4 ± 0.3	—	15.8 ± 0.3	5.5 ± 0.5	804 ± 90a	[77]
ZrB2 + 10HfB2 + 15SiC	98.2	3	—	508 ± 4	18.2 ± 0.5	4.1 ± 0.8	765 ± 75	[52]
ZrB2 + 35HfB2 + 15SiC + 4.5ZrN	99.5	—	—	494 ± 4	16.7 ± 0.7	4.8 ± 0.2c	590 ± 25	[72]
ZrB2 + 9.6SiC + 28.9ZrC + 3.7Si3N4	99.5	2	—	450	21.1 ± 0.8	3.8 ± 0.1	510 ± 160	[67]
ZrB2 + 18.5SiC + 3.7Si3N4 + 1Al2O3+ 0.5Y2O3	98	2.5 ± 0.1	—	421 ± 5	14.2 ± 0.6	4.6 ± 0.1	710 ± 110	[11]

^aThree-point flexure.

^bZrB₂ basis.

^cChevron notched beam.

^dSingle-edge notched beam.

^eSiC basis.

^fIndentation strength in bending.

In contrast to oxide and nitride additions, the mechanical properties of ZrB₂–SiC ceramics with ZrC (Table 8.4) typically result in mechanical properties similar to ZrB₂–SiC without ZrC additions (Table 8.3). This is most likely the result of closer CTE match between ZrC (7.6 × 10⁻⁶ K⁻¹) [78] and ZrB₂ compared to SiC and ZrB₂, resulting in the SiC phase still dominating the mechanical behavior. With ZrC additions from 5 to 30 vol%, the flexure strengths (~500–750 MPa), fracture toughnesses (3.5–6.5 MPa · m^½) are nominally the same as ZrB₂–SiC. Similarly, it is reasonable to assume that the same microstructural controls used in ZrB₂–SiC to improve mechanical properties can be employed in the ZrB₂–SiC–ZrC system. Small additions of other carbides (C, B₄C, VC, and WC) have little effect on the mechanical properties of ZrB₂–SiC. However, these additions improve densification of ZrB₂–SiC in the same manner as when added to ZrB₂, through removal of surface oxides on the starting powders. Thus, small additions of carbides can also be used to improve the room temperature mechanical properties by allowing greater control of microstructure during sintering.

8.2.3 ZrB₂ with Disilicide Additions

8.2.3.1 Flexure Strength.

The flexure strength of ZrB₂ with additions of ZrSi₂, MoSi₂, or TaSi₂ is summarized in Table 8.5. Figure 8.8 shows the effect of MoSi₂ and ZrSi₂ additives on the strength of ZrB₂. Guo [17] reported strengths for ZrB₂–ZrSi₂ in the range of 380 to 555 MPa depending on ZrSi₂ content. Chamberlain [27] reported the effect of MoSi₂ on strength of ZrB₂, increasing from 565 MPa for ZrB₂ to 1150 MPa for 10 vol% MoSi₂, decreasing to approximately 1020 MPa for 20 and 30 vol% additions of MoSi₂. Similarly, Guo [64] also reported the strength of ZrB₂ to increase from 460 MPa for ZrB₂ [17] to between 750 and 800 MPa for 10–40 vol% MoSi₂ additions.

Table 8.5. Elastic modulus, Vickers hardness, fracture toughness by direct crack method, and four-point flexure strength of ZrB₂–(Zr, Mo, Ta)Si₂ ceramics

Composition	Relative density	ZrB2 Grain size	MeSi2 Grain size	Elastic modulus	Hardness	Fracture toughness	Flexure strength	References
(vol%)	(%)	(µm)	(µm)	(GPa)	(GPa)	(MPa · m½)	(MPa)
ZrB2 + 10ZrSi2	96.6	2.3 ± 0.9	0.6 ± 0.4	432	—	3.8 ± 0.3	483 ± 22	[17]
ZrB2 + 20ZrSi2	99.1	2.5 ± 0.8	0.7 ± 0.6	445	—	4.4 ± 0.5	556 ± 54	[17]
ZrB2 + 30ZrSi2	99.8	2.6 ± 1.1	0.7 ± 0.6	427	—	4.4 ± 0.2	555 ± 42	[17]
ZrB2 + 40ZrSi2	99.2	2.7 ± 1.0	0.9 ± 0.7	397	—	3.9 ± 0.4	382 ± 78	[17]
ZrB2 + 2.3MoSi2	6.04 g/cm3	5	—	500 ± 2	18.1 ± 0.4	3.4 ± 0.3a	750 ± 160	[14]
ZrB2 + 5MoSi2	96	2.6	—	516 ± 4	15.2 ± 1.0	2.9 ± 0.1a	569 ± 54	[79]
ZrB2 + 10MoSi2	99.7	1.9 ± 0.6	1.8 ± 0.5	490 ± 7	15.8 ± 0.7	3.7 ± 0.3	800 ± 108	[64]
ZrB2 + 10MoSi2	6.19 g/cm3	~3	—	516	20.4 ± 2.2	4.1 ± 0.4b	1151 ± 52	[27, 28]
ZrB2 + 10MoSi2	99.9	—	—	—	17.5 ± 0.4	3.8 ± 0.4	560 ± 115c	[80]
ZrB2 + 15MoSi2	5.99 g/cm3	1.9 ± 0.6	—	452 ± 4	14.7 ± 0.6	3.5 ± 0.6a	780 ± 87	[66]
ZrB2 + 15MoSi2	98.1	1.4	—	479 ± 4	16.2 ± 0.5	2.6 ± 0.3	643 ± 97	[65–67]
ZrB2 + 20MoSi2	99.9	—	—	—	14.9 ± 0.1	3.2 ± 0.2	680 ± 40c	[80]
ZrB2 + 20MoSi2	6.17 g/cm3	~3	—	523	18.5 ± 2.7	3.0 ± 0.2b	1008 ± 137	[27, 28]
ZrB2 + 20MoSi2	99.8	1.6 ± 0.6	2.7 ± 0.9	472 ± 6	16.3 ± 0.9	2.8 ± 0.2	750 ± 128	[64]
ZrB2 + 20MoSi2	99.1	~2.5	—	489 ± 4	16.0 ± 0.4	2.3 ± 0.2	531 ± 46	[65, 79, 81, 82]
ZrB2 + 30MoSi2	99.7	—	—	—	14.3 ± 0.2	3.2 ± 0.3	550 ± 65c	[80]
ZrB2 + 30MoSi2	99.8	2.1 ± 0.7	2.4 ± 0.6	473 ± 3	15.4 ± 0.7	2.6 ± 0.2	757 ± 76	[64]
ZrB2 + 30MoSi2	6.29 g/cm3	~3	—	494	17.7 ± 1.6	4.0 ± 0.4b	1031 ± 150	[27, 28]
ZrB2 + 40MoSi2	99.7	1.9 ± 0.7	2.6 ± 0.8	448 ± 4	13.2 ± 0.7	3.1 ± 0.3	790 ± 57	[64]
ZrB2 + 15TaSi2	99	2	—	444 ± 24	17.8 ± 0.5	3.8 ± 0.1a	840 ± 33	[83]

^aChevron notched beam.

^bIndentation strength in bending.

^cThree-point flexure.

Figure 8.8. Room-temperature flexure strength and fracture toughness as a function of disilicide concentration for ZrB₂–MeSi₂ ceramics produced by hot pressing [17, 27, 64].

The strength of ZrB₂ with MoSi₂ additions has been reported by numerous researchers. This has resulted in a wide range of reported strengths: ZrB₂–10 vol% MoSi₂, 560–1150 MPa; ZrB₂–20 vol% MoSi₂, 460–1010 MPa; ZrB₂–30 vol% MoSi₂, 550–1030 MPa. These values highlight the importance of microstructure and processing control similar to the ZrB₂–SiC system. Unfortunately, microstructural effects of disilicide additions on the strength of ZrB₂ have not been thoroughly investigated. This is further complicated by the tendency of the ZrB₂–disilicide systems to form complex grain boundary phases and solid solution phases with the ZrB₂. Silvestroni et al. [84] reported the formation of a ZrB₂ core–(Zr,Mo)B₂ rim structures during sintering. In addition, ZrC, ZrO₂, MoB, and SiO₂ were detected in the final microstructures. Further, they observed various Mo–Zr–B–Si phases that may have also contained C and O, as well as the core–rim structures. These complex chemistries and architectures make analysis of microstructure–mechanical properties' effects difficult. Though detailed microstructural analysis has not been performed on the other transition metal disilicide systems, similar processes presumably occur in these systems as well.

8.2.3.2 Fracture Toughness.

Table 8.5 shows the fracture toughness of ZrB₂ with the addition of ZrSi₂, MoSi₂, or TaSi₂. The effect of ZrSi₂ and MoSi₂ additions on the fracture toughness of ZrB₂ is also shown in Figure 8.8. In general, MoSi₂ additions result in fracture toughness values in the range of 3.2–4.0 MPa · m^½. Chamberlain [27] reported indentation strength in bending (ISB) fracture toughness values of 3–4 MPa · m^½ for ZrB₂–MoSi₂, but without a trend with MoSi₂ content. Guo [64] measured fracture toughness of ZrB₂–MoSi₂ using the direct crack method (DCM) and reported values that decreased from 4.8 MPa · m^½ for ZrB₂ [17] to approximately 2.8 MPa · m^½ for additions of 20–40 vol% MoSi₂. Guo [17] also reported DCM toughness for ZrB₂–ZrSi₂ as 3.8 MPa · m^½ for 10 vol% additions, increasing to 4.4 MPa · m^½ for 20 and 30 vol% additions, and then decreasing to 3.9 MPa · m^½ for 40 vol% ZrSi₂ additions. Unlike SiC additions to ZrB₂, no trend was observed for fracture toughness with disilicide additions.

Using the Griffith criteria and a Y-parameter of 1.99 (long, semi-elliptical surface flaw), the ZrB₂–MoSi₂ composites produced by Chamberlain and Guo both exhibit critical flaw sizes that correlated to the observed grain sizes. For example, Chamberlain's ZrB₂–10 vol% MoSi₂ has a calculated flaw size of 3.2 μm and a reported grain size of approximately 3 μm. However, Guo observed that the calculated critical flaw size was much larger than the observed grain size, suggesting that something other than grain size was controlling the strength of the ZrB₂–ZrSi₂ composites. From the microstructural analysis of polished cross sections and fracture surfaces, Guo observed that the ZrSi₂ segregated to the grain boundaries during sintering, exhibiting a “string of pearls” type morphology and clustering of ZrSi₂ grains. Further, small amounts of porosity were observed in the ZrSi₂ clusters between the grains. For ZrB₂–ZrSi₂, the size and morphology of the ZrSi₂ phase controls the failure behavior of the composite.

8.2.3.3 Elastic Modulus.

Table 8.5 also summarizes the hardness and elastic modulus of ZrB₂ with the addition of ZrSi₂, MoSi₂, or TaSi₂. The effect of additive concentration on the elastic modulus of ZrB₂–MoSi₂ and ZrB₂–ZrSi₂ is shown in Figure 8.7. The dashed lines represent the expected modulus based on linear volumetric rule of mixtures calculations assuming moduli of 510 GPa for ZrB₂, 440 GPa for MoSi₂ [85], and 235 GPa for ZrSi₂ [86]. From Figure 8.7, it can be seen that the ZrB₂–MoSi₂ composites follow the rule of mixtures trend, but the values are consistently 10–20 GPa lower than expected. This is likely due to the formation of complex phases and solid solutions during densification ZrB₂–MoSi₂ composites. Thermal residual stresses resulting from the CTE mismatch between ZrB₂ (α_a-axis = 6.9 × 10⁻⁶ K⁻¹, α_c-axis = 6.7 × 10⁻⁶ K⁻¹) [49] and MoSi₂ (α_a-axis = 8.2 × 10⁻⁶ K⁻¹, α_c-axis = 9.4 × 10⁻⁶ K⁻¹) [87] may also play a role in the observed difference between the predicted and observed modulus. In the case of ZrB₂–ZrSi₂, the observed modulus is in good agreement with the predicted modulus. Since a tendency to form complex phases has not been observed in the ZrB₂–ZrSi₂ system, this likely explains why the elastic modulus of ZrB₂–ZrSi₂ follows predictions while ZrB₂–MoSi₂ has a more complex behavior.

8.2.4 ZrB₂–MeSi₂–SiC

The flexure strength and fracture toughness of several ZrB₂–disilicide ceramics with SiC additions is shown in Figure 8.9. Guo [64] investigated the effect of SiC additions to ZrB₂–20 vol% MoSi₂ and ZrB₂–40 vol% MoSi₂ ceramics. The strength of both ZrB₂–20 vol% MoSi₂ and ZrB₂–40 vol% MoSi₂ increased with the addition of 5 vol% SiC, but decreased with SiC contents of more than 5 vol%. Further, toughness increased with the addition of SiC, but was relatively insensitive to the amount. The improvement in properties was the result of a finer grain size for the ZrB₂ and MoSi₂, with the addition of 5 vol% SiC. Further additions of SiC increased the observed average and maximum grain sizes for ZrB₂, MoSi₂, and SiC. A Griffith-type failure analysis revealed that the critical flaw size increased with the SiC content, and is consistent with the maximum observed MoSi₂ grain size. Elastic modulus increased with a 5 vol% SiC addition, then decreased with further additions. Additions of SiC to ZrB₂–TaSi₂ [88, 89] resulted in similar trends in mechanical properties as additions to ZrB₂–MoSi₂. Additions of SiC to ZrB₂–5 vol% TaSi₂ and ZrB₂–10 vol% TaSi₂ increased the toughness. In general, additions of SiC to ZrB₂ with disilicides results in improved toughness.

Figure 8.9. Room-temperature flexure strength and fracture toughness as a function of SiC content for ZrB₂–MoSi₂–SiC and ZrB₂–TaSi₂–SiC ceramics [64, 88, 89].

8.3 Elevated-Temperature Mechanical Properties

While several studies have reported elevated-temperature mechanical properties of ZrB₂ ceramics, most have only reported a single property (i.e., strength) for a limited number of temperatures and have not provided a systematic evaluation of the effects of temperature on mechanical behavior. The following discussion highlights studies of the elevated-temperature elastic modulus, strength, and fracture toughness of various ZrB₂-based UHTCs.

8.3.1 Elastic Modulus of ZrB₂-Based Ceramics

The elevated-temperature elastic modulus of hot-pressed ZrB₂ with and without additives is shown in Figure 8.10. Only limited data on elastic behavior of ZrB₂ have been published for elevated temperatures. Okamoto et al. [49] reported elastic modulus from single crystal values as being 525 GPa at room temperature, decreasing linearly to 490 GPa at 1100°C. Experimental results of bulk ZrB₂ have resulted in lower values of elastic modulus than those reported by Okamoto. The historical work of Rhodes et al. [25] provided the only reported values for the elastic modulus of bulk ZrB₂ tested in inert atmosphere. The modern studies by Zhu [20] and Neuman et al. [29] were conducted in air, and thus are affected by the formation of an oxide scale on the test articles. Unfortunately, the effect of the oxide scale on the measured elastic modulus was not investigated. Regardless, the elastic modulus of bulk ZrB₂ decreased more rapidly than expected based on single crystal measurements presumably due to a grain boundary effect. The modulus decreased linearly from room temperature to approximately 1200°C (450 GPa). Above this temperature, the elastic modulus decreased more rapidly with temperature. The change in slope was thought to be a result of softening of grain boundary phases combined with the activation of grain boundary sliding and diffusional creep mechanisms [20, 25, 29, 69, 90]. Reported modulus values were similar up to 1600°C (250 MPa), despite the differences in testing methods. To date, Rhodes [25] is the only study to report the modulus above 1600°C, where it decreases to 100 GPa at 2000°C.

Figure 8.10. Elevated-temperature elastic modulus of hot-pressed ZrB₂ with and without additives [20, 25,29].

The elevated-temperature elastic modulus of ZrB₂–SiC ceramics is shown in Figure 8.11. Rhodes et al. [25] observed a steady decrease in modulus from 530 GPa at room temperature to 420 GPa at 1400°C. Above 1400°C, the modulus decreased dramatically to approximately 100 GPa at 1600°C. Neuman et al. [69] observed a steady decrease from 510 GPa at room temperature to 410 GPa at 1000°C. The slope of the modulus then changed, decreasing to 210 GPa at 1500°C, and then more rapidly to 110 GPa at 1600°C. As with ZrB_2, the changes in slope of the elastic modulus are likely the result of softening of grain boundary phases, and grain boundary sliding and diffusional creep. In ZrB₂–SiC, the softening was expected to occur at a lower temperatures to the presence of SiO₂ (surface oxide impurity from SiC) and its interaction with the B₂O₃ and ZrO₂ present from the ZrB₂. Zou et al. [90] measured the internal damping in ZrB₂–SiC prepared by milling with Si₃N₄ balls, and found that the damping peaks just above 800°C and again around 1400°C. The reduction in modulus with temperature for ZrB₂–SiC ceramics is enhanced by the presence of oxides at the grain boundaries and triple junctions as well as the polycrystalline nature of the material.

Figure 8.11. Elevated-temperature elastic modulus of hot- pressed ZrB₂–SiC with and without additives [25, 69].

8.3.2 Strength and Fracture Toughness

8.3.2.1 Strength of ZrB₂.

 The elevated-temperature four-point flexure strength of ZrB₂ with and without additives is shown in Figure 8.12. In the historical study of Rhodes [25], a room-temperature strength of 325 MPa was measured for fully dense ZrB₂ with an approximately 20 μm grain size. Strength increased between room temperature and 800°C (420 MPa), as a result of relief of thermal residual stresses. Strength decreased to 145 MPa at 1400°C, increased to 200 MPa at 1900°C, and then decreased to 50 MPa at 2200°C. The observed increase in strength from 1400 to 1900°C was thought to be caused by stress relief during testing through plastic flow, arising from diffusional creep. A general trend was observed that finer grain size, and lower porosity resulted in improved elevated-temperature strength, which was offset by enhanced creep in the finer-grained material. Similar results have been observed in other structural ceramic systems such as SiC, ZrO₂, Al₂O₃, and Si₃N₄.

Figure 8.12. Elevated-temperature flexure strength of selected hot-pressed ZrB₂ ceramics with and without additives in air and argon [11–13, 20, 25, 29, 30].

Since the historical work of Rhodes [25], several modern studies have investigated the strength of ZrB₂ at elevated temperatures. Melendez-Martinez et al. [12] measured the flexure strength of an 87%-dense ZrB₂ (350 MPa at room temperature) up to 1400°C in air (220 MPa). Zhu [20] produced dense ZrB₂ and ZrB₂–2 wt% B₄C–1 wt% C with grain sizes of 8.1 and 2.5 μm, respectively. At room temperature, the ZrB₂ had a strength of 450 MPa compared to 575 MPa for ZrB₂–B₄C–SiC. The ZrB₂ strength increased up to 1200°C (500 MPa) before decreasing to 360 MPa at 1500°C. The strength of ZrB₂–B₄C–C increased to 630 MPa at 800°C, then steadily decreased to 370 MPa at 1500°C. Zhu performed his testing in air, and even though he used a protective SiO₂ coating, the strength above 1200°C was likely controlled by flaws induced by oxidation damage (discussed in more detail later). For dense ZrB₂ ceramic with 0.5 wt% carbon added as sintering aid and a grain size of approximately 19 μm [29] the room temperature strength of the modern ZrB₂ was 380 MPa, compared to 325 MPa for the Rhodes material. The strength was maintained up to 1200°C before decreasing to a minimum of 170 MPa at 1500°C. Above 1600°C, the strength was approximately 220 MPa up to 2300°C. Chemical compatibility between ZrB₂ and the graphite test fixture (T_e = 2390°C) prevented testing at higher temperatures. The improvement in strength, especially above 2000°C, was most likely due to improved purity of modern ZrB₂ powders (the ZrB₂ used by Rhodes contained several percent ZrC and ZrO₂). Additionally, the carbon sintering aid helped remove the surface oxides present on the ZrB₂ starting powders [22], presumably leaving less oxide phase at the grain boundaries, which would improve high-temperature strength and stiffness.

8.3.2.2 Strength of ZrB₂–SiC.

 The elevated-temperature flexure strength of several hot-pressed ZrB₂–SiC ceramics with and without additives is shown in Figure 8.13. The ZrB₂–20 vol% SiC studied by Rhodes [25] exhibited an increase in strength from 390 MPa at room temperature to 420 MPa at 800°C. The strength steadily decreased to 245 MPa at 1800°C and decreased more rapidly to 115 MPa at 2000°C. Modern ZrB₂–SiC exhibited higher strengths at room and elevated temperature compared to Rhodes' material and similarly for all temperatures measured thus far. Zou et al. [90, 91] have reported the three-point bend strengths at elevated temperature for several compositions up to 1600°C in argon. Zou reported a strength of 550 MPa for ZrB₂–20 vol% SiC at room temperature, increasing to 680 MPa at 1000°C, then steadily decreasing to 460 MPa at 1600°C. Zou added 5 vol% WC, which resulted in a room-temperature strength of 605 MPa that steadily increased to 675 MPa at 1600°C. Interestingly, the composition ZrB₂–20SiC–5ZrC (not shown) exhibited a room-temperature strength of 1100 MPa, only slightly decreasing to 1020 MPa at 1000°C. Increasing temperatures resulted in a steep decrease in strength to 320 MPa at 1600°C. Grigoriev [15] measured the strength of a ZrB₂–19 vol% SiC composite in air (not shown), reporting a room-temperature strength of 500 MPa that was maintained up to 1200°C, decreasing to 430 MPa at 1400°C. Not only do the room-temperature strengths vary widely for ZrB₂–SiC, but the change in strength with temperature also varies.

Figure 8.13. Elevated-temperature flexure strength of selected hot-pressed ZrB₂–SiC ceramics with and without additives in argon [25, 69, 90, 91].

Zou showed that the addition of WC to ZrB₂–20SiC resulted in finer ZrB₂ and SiC grain sizes plus enhanced removal of surface oxides from the starting powders. Reducing the amount of low-melting-point oxides present at the grain boundaries should result in a higher stiffness at elevated temperatures, thus also improving strength and resistance to creep. Similarly, the increase in strength for ZrB₂–20SiC–5ZrC at room temperature was attributed to grain refinement. The steep drop in strength at elevated temperatures was attributed to the softening of residual oxides present at grain boundaries, since no favorable reactions with the surface oxides or ZrC were identified up to the sintering temperature. Further, creep was observed in the material during testing at 1600°C. Hence, the presence of oxide impurities reduces strength and increases creep at elevated temperatures.

Neuman et al. reported the elevated-temperature four-point flexure strengths of ZrB₂–30SiC with 2 wt% B₄C in both an inert argon atmosphere and air (not shown) [30, 69]. The strength of ZrB₂–30SiC–2B₄C in argon was 680 MPa at room temperature, steadily decreased to 540 MPa at 1800°C, then more rapidly decreased to 450 MPa at 2000°C, and finally reached 275 MPa at 2200°C. In air, the strength increased from 680 MPa at room temperature to approximately 740 MPa at 800 and 1000°C and decreased to approximately 370 MPa at 1500 and 1600°C. The increase in strength between room temperature and 1000°C was attributed to the healing of surface flaws from the formation of the ZrO₂–B₂O₃ oxide scale. The size of the SiC clusters controlled the strength up to 1800°C in the inert atmosphere. In air, the size of the SiC clusters controlled strength up to 1000°C, while oxidation damage controlled strength above 1200°C, as discussed in the following. The elevated temperature testing performed by Neuman was different than other studies in that increasing testing rates were used at increasing temperatures to stay in the fast fracture regime and avoid creep effects, which is consistent with the methods described in ASTM C1211. The increased testing rate typically results in higher measured strengths compared to tests using a slower rate where nonlinear-elastic behavior is observed.

Figure 8.14 also shows the elevated-temperature flexure strengths of some ZrB₂–SiC ceramics with various additives. Rhodes added graphitic carbon to ZrB₂–SiC to improve the thermal shock resistance. The strength increased from 520 MPa at room temperature to 640 MPa at 600°C in argon and then steadily decreased to 260 MPa at 1800°C. Monteverde [11] added 4 vol% Si₃N₄ as a sintering aid to ZrB₂–20 vol% SiC. The strength decreased from 730 MPa at room temperature to 250 MPa at 1200°C due to formation of various phases containing Zr, Si, and/or B with O and/or N at the grain boundaries and triple junctions. These phases begin to soften at temperatures as low as 800°C, leading to strength degradation with increasing temperatures. Bellosi [67] measured the strength of a ZrB₂–10SiC–30ZrC composite in argon, showing a room-temperature strength of 720 MPa, which steadily decreased to 420 MPa at 1500°C. The decline in strength between 1200 and 1500°C was similar to the decline observed by Zou over a similar temperature range. Grigoriev made additions of 4 and 7 vol% ZrSi₂ to ZrB₂–18SiC and ZrB₂–17SiC. In the case of the 4% addition, a strength of approximately 480 MPa was maintained up to 1200°C, before decreasing to 230 MPa. Adding 7 vol% ZrSi₂ resulted in a decrease in strength from 400 MPa at room temperature to approximately 230 MPa at 1200 and 1400°C. Grigoriev did not speculate as to the effect of ZrSi₂ on the response of strength at elevated temperatures, but it is likely due to the relatively low melting point of ZrSi₂ of 1620°C and the formation of complex phases at the grain boundaries and triple junctions as seen with in ZrB₂–MoSi₂ as discussed in the following.

Figure 8.14. Elevated-temperature flexure strength of selected hot-pressed ZrB₂–SiC ceramics with various additives in argon [11, 15, 25, 67].

8.3.2.3 Strength of ZrB₂ with MoSi₂ and TaSi₂.

 Figure 8.15 shows the four-point flexure strengths at elevated temperatures in air for several ZrB₂ ceramics containing MoSi₂ or TaSi₂. ZrB₂–20 vol% SiC prepared by Rhodes is included as a comparison. Silvestroni [79] measured the strength of a pressurelessly sintered ZrB₂–5 vol% MoSi₂ ceramics, finding that the strength decreased only slightly from 570 MPa at room temperature to 490 MPa at 1500°C. Bellosi et al. [67] reported properties for a spark-plasma-sintered ZrB₂–15MoSi₂ ceramic, finding that the strength remained approximately 635 MPa from room temperature up to 1200°C, before decreasing to 360 MPa at 1600°C. In a follow-up study, Balbo and Sciti [66] reported similar results for a hot-pressed material of the same composition, speculating that the decrease in strength at 1500°C was the result of softening of silicate phases present in the material. Sciti et al. [81] also measured the strength of pressurelessly sintered ZrB₂–20MoSi₂ ceramics, in this case observing an increase in strength from 530 MPa at room temperature to 655 MPa at 1200°C, declining to 500 MPa at 1500°C. They attributed the increase in strength at 1200°C to healing of surface flaws due to formation of an oxide scale. Sciti et al. [83] also investigated the properties of hot-pressed ZrB₂–TaSi₂, reporting a strength of 840 MPa at room temperature that decreased to 375 MPa at 1500°C. This strength was slightly higher than they observed for a comparably processed ZrB₂–15MoSi₂ (705 MPa at room temperature, 335 MPa at 1500°C), but the strength followed a similar trend. The increased strength of ZrB₂–TaSi₂ was likely a result of the observed increase in fracture toughness between the MoSi₂- and TaSi₂-containing composites, but no further speculation was offered by the authors.

Figure 8.15. Elevated-temperature four-point flexure strength of selected ZrB₂–MeSi₂ ceramics in air [25, 65–67, 79, 82, 83].

The ZrB₂–transition metal disilicide systems show promise for their retention of strength at elevated temperatures. Unfortunately, few data are available for these systems above 1500°C. In addition, no elastic properties or fracture toughness behavior have been reported at elevated temperatures. Chemical stability issues may also restrict the use of these composites in the ultra-high temperature regimes. These are discussed in more detail later, but should not be meant to diminish the promising properties these materials have demonstrated thus far.

8.3.2.4 Fracture Toughness of ZrB₂ and ZrB₂–SiC.

 Measuring the fracture toughness allows for the critical flaw size for failure to be calculated using the measured strength. Figure 8.16 shows the fracture toughness of ZrB₂ [30] and ZrB₂–30 vol% SiC [69] determined by chevron notched beam in flexure at elevated temperatures. The fracture toughness of ZrB₂ with 0.5 wt% C increased from 2.9 MPa · m^½ at room temperature to 5.2 MPa · m^½ at 1200°C, above which toughness steadily decreased to 3.7 MPa · m^½ at 2300°C. Using the Griffith criterion and a Y-parameter of 1.29 (semicircular surface crack), the critical flaw size was similar to the maximum grain size for nearly all temperatures. The critical flaw size calculated for the material at 1400°C was slightly higher than any features observed in the microstructure. The cause of this discrepancy is currently unknown, but could be due to a change in the stress state, since Watts [56] found that stresses begin to accumulate in ZrB₂ below 1400°C and Rhodes [25] observed plasticity at 1400°C and higher.

Figure 8.16. Elevated-temperature fracture toughness (CNB) of hot-pressed ZrB₂ and ZrB₂–SiC ceramics [30, 69].

The fracture toughness of ZrB₂–30SiC with 2 wt% B₄C, was measured up to 1600°C in air [69]. The fracture toughness was approximately 4.8 MPa · m^½ from room temperature to 800°C, steadily decreasing to 3.3 MPa · m^½ at 1600°C. Failure analysis revealed that SiC clusters acted as critical flaws below 1000°C, while oxidation damage was the critical flaw above 1200°C. While the oxide scale is a logical choice for failure origin in these materials, the study by Neuman showed that regions of enhanced ingress of oxide scale were the critical flaws. Thus, control of the microstructure is only part of the solution to improving the performance of these ceramics at elevated temperatures in oxidizing atmospheres. Control of the oxidation behavior and the morphology of the oxide scale are also critical for improving their performance in structural applications in extreme environments.

Kalish et al. [92] reported an increase in the amount of transgranular fracture in ZrB₂ from approximately 20% at room temperature up to approximately 60% at 1000°C. The amount of transgranular fracture then decreased to approximately 10% at 1200 and 1400°C. Similarly, Bird et al. [60] observed an increase in the amount of intergranular fracture in ZrB₂–20 vol% SiC from approximately 5% at room temperature to approximately 95% at 1400°C. The amount of intergranular fracture increased between 800 and 1000°C. The increase in the amount of intergranular fracture with temperature corresponded to an increase in the observed plateau in toughness, which increased from approximately 2.8 MPa · m^½ at room temperature to approximately 5.8 MPa · m^½ at 1400°C.

8.4 Concluding Remarks

This chapter presented a review of the current state-of-the-art of the mechanical properties of zirconium diboride-based ceramics. These ceramics offer a combination of properties that include high refractoriness, modulus, hardness, strength, and moderate fracture toughness. Their mechanical properties are controlled by their microstructures. The control of grain size, the dispersion and cluster size of second phases, and impurity phases are critical for the production of materials for use in extreme environment applications. Lack of sufficient control over the microstructure and impurities reduces the properties of these materials—for example, exceeding the microcracking threshold. The mechanical properties of these materials at elevated temperatures were also reviewed. To date, there has been a lack of fundamental research studies of the mechanical properties of these materials at elevated temperatures. Since the main driver for research into the ZrB₂ system is for use in ultra-high temperature (>2000°C) applications, further effort is needed to evaluate the mechanical properties of these materials in relevant environments. It was shown that there is currently a lack of robust microstructure—property relations for the ZrB₂-based systems. Microstructure and impurity characterization needs to be performed, and controlled, in future studies.

This chapter has shown that ceramics based on ZrB₂ offer promise for use as structural materials in extreme environments. It was shown that ZrB₂ ceramics offer mechanical properties at ultra-high temperatures that should allow designers to incorporate these materials for use in future applications for hypersonic vehicles and in other industrial applications where ceramics that can maintain strength (hundreds of MPa) at temperatures exceeding 1500°C are needed.

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Note

a Current affiliation: Intel Corporation, Beaverton, OR, USA.