Engineered Role of SiC Particle Size on Multi‐Length‐Scale Wear Damage of Spark Plasma Sintered Zirconium Diboride

Abstract

Among all the members from different families of ultra-high temperature ceramics (UHTCs), including borides, carbides, and nitrides, zirconium diboride (ZrB 2) is a special one, [1] possessing extraordinary combination of properties, [2-4] such as high mechanical properties viz. hardness (15-29.4 GPa [5-8]), fracture toughness (2.8-4 MPa m 1/2[6,9,10]), high thermal conductivity (57.9-60 W m À1 K À1[11,12]), low electrical resistivity (9.2-10 μΩ cm [13,14]), and also low density (6.1 g cm À3[7,15]). However, for hypersonic applications, hardness, fracture toughness, and tribological properties need to be improved, and the density of monolithic ZrB 2 needs to be reduced further to produce maneuverable hypersonic vehicles. [16] Moreover, poor sinterability of UHTCs, in general, and, thus, in ZrB 2 , due to its strong covalent bonding, adherence of oxide impurities on its particles' surface, and poor self-diffusivity, has always been a matter of concern. [17,18] In this context, reinforcement with a suitable secondary phase, such as SiC, B 4 C, ZrC, and MoSi 2 , has improved not only densification but also mechanical properties, such as hardness and fracture toughness, oxidation resistance, thermal properties, and wear resistance as well. [16,18-21] Among these reinforcements, B 4 C and SiC possess low density (2.52 and 3.16 g cm À3 , respectively [16,22]) and are attracting more interest in enhancing the performance of UHTCs. Enormous research has been carried out to study the effect of reinforcements on the mechanical, oxidation, and thermal performance of ZrB 2. SiCreinforced ZrB 2 composites have received special interest, [23-25] considered as baseline UHTC composites [17,26-29] with 20 vol% SiC as optimal for aerospace applications; [30-33] however, studies regarding the effect of reinforcement on tribological behavior are available in scarce as compared with studies on other aspects, such as mechanical, thermal, and oxidation behavior. One of the earliest studies on the friction and wear of hot pressed (2100 C for 1 h at 34.3 MPa in Ar) ZrB 2-based composites carried out by Umeda et al. [34] in the 1990s showed the coefficient of friction (cof) varied between 0.90 and 0.95. With an increase in relative humidity (RH; from <10 to >70%), cof decreased to 0.2-0.4. The tests were performed in air using reciprocating pin-on-block instrument with pin and block of the same material at 7.8 N load and a sliding speed of 1.5 mm s À1 for 100 cycles. Hertzian cracks were formed perpendicular to the sliding direction in ZrB 2 and ZrB 2 þ B 4 C (50:50 wt%) composite, whereas no cracks developed in the ZrB 2 þ B 4 C þ SiC (46:46:8 wt%) composite, revealing the effective role of synergetic reinforcement of B 4 C and SiC in toughening the ZrB 2 matrix. Recently, Medveď et al. [35] processed ZrB 2-based composites viz. ZrB 2 þ 10 wt% B 4 C, ZrB 2 þ 10 wt% SiC, and ZrB 2 þ 10 wt% ZrC via spark plasma sintering (SPS; sintered from 1800 to 2050 C at 50 MPa for 10 min in Ar) to study the tribological behavior of carbide reinforceme...