Dynamic Indentation Response of Fine-Grained Boron Carbide Dipankar Ghosh Department of Materials Science and Engineering, Michigan Technological University, Houghton, Michigan 49931-1295 Ghatu Subhash* ,w Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, Florida 32611 Tirumalai S. Sudarshan and Ramachandran Radhakrishnan* Materials Modifications Inc., Fairfax, Virginia 22031 Xin-Lin Gao Department of Mechanical Engineering, Texas A&M University, College Station, Texas 77843-3123 Boron carbide disks with three different grain sizes were con- solidated from submicrometer-sized boron carbide powder using the plasma pressure compaction technique. Static and dynamic indentations were performed to determine their loading-rate dependence on mechanical properties. Dynamic indentations resulted in a decrease in hardness and fracture toughness, and induced more severe damage compared with static indentations. Using Raman spectroscopy, the mechanism responsible for loss of strength under dynamic loads was identified as the solid-state structural phase transformation in the dynamically loaded re- gions. The influence of processing conditions and the resulting microstructure on the observed rate dependency of mechanical properties are discussed. I. Introduction B ORON carbide (B 4 C) ceramic is an excellent candidate mate- rial for structural applications 1–7 at room and high temper- atures because of its high melting point (24501C), high elastic modulus (450 GPa), high hardness (HV 25–35 GPa, next only to diamond and cubic boron nitride), high flexural strength (350– 500 MPa), low density (2.52 g/cm 3 ), and excellent wear resis- tance. It is used as a grinding medium for hard materials, as a lightweight ceramic armor, as wear-resistant sandblasting nozzle material, and as a neutron absorber in nuclear reactors. 1,8 How- ever, the processing of boron carbide is challenging due to the difficulty associated with sintering of the starting boron carbide powder. Traditionally, boron carbide has been consolidated us- ing (i) hot pressing with and without sintering additives, 1,9,10 (ii) hot isostatic pressing (HIP), 1,11 (iii) pressureless sintering with sintering additives, 1,2,4–6,12,13 (iv) pressureless sintering in a gaseous atmosphere of hydrogen and helium, 14 and (v) micro- wave sintering. 15 Among the above processing techniques, hot- pressing and pressureless sintering are the most commonly used methods to produce boron carbide ceramics with 95%–99% of the theoretical density and with grain sizes in the range between 1.5 and 60 mm. However, in these processing methods, the sintering temperatures were relatively high (420001C) and the sintering times were on the order of hours. In addition, sintering aids were found to reduce fracture strength moderately, 13 and the resulting sintered ceramics were not suitable for nuclear applications 1 where high-purity boron carbide is required for neutron absorption. A dense boron carbide ceramic can be pro- duced at lower temperatures using HIP but this method is not suitable for bulk processing. 1,11 Boron carbide powder heat treat- ed in a gaseous mixture of hydrogen and helium and then sintered in the presence of pure helium also requires a sintering temper- ature above 22001C. 14 Microwave sintering has also been used to consolidate boron carbide (95% density) in a short duration of time (12 min) but it also requires high temperatures around 20001C. 15 Recently, Klotz et al. 16 applied a novel non-conventional method known as plasma pressure compaction (P 2 C s ) for bo- ron carbide consolidation. Sintering was performed at 16501C within a short consolidation time of 5 min, but the resulting density was only around 91% of the theoretical value. Several sintering additives such as graphite, alumina, and titanium dibo- ride were used to achieve up to 97% of theoretical density. In the current work, boron carbide was consolidated using the above P 2 C s method with an intent to produce theoretically dense compacts with various grain sizes but without the addition of sintering aids. Investigations on the evaluation of the mechanical properties of boron carbide have been mainly focused on determining elas- tic modulus, flexural strength, fracture toughness, 1–7 and nano- indentation response. 17,18 Owing to its promise as an effective armor material against low-velocity ballistic threats, 1,8 high strain rate experiments were conducted on boron carbide. How- ever, these experiments are expensive, time consuming, and re- quire large-size specimens. 8,19–21 Therefore, in the current work, dynamic indentation experiments at high loading rates were conducted to capture the strain rate sensitivity of indentation response in boron carbide ceramics of different grain sizes. A novel dynamic indentation tester 22–24 was utilized to determine the dynamic hardness and then to compare these results with the static hardness measurements. To study the damage accumulat- ed beneath the indentation, subsurface studies using the bonded- interface technique was conducted. 25 Raman spectroscopy is a useful nondestructive tool for analyzing structural changes as well as phase transformations in crystalline materials. 26 This technique has been used to investigate the contact damage in- duced by nanoindentation in single crystal and polycrystalline boron carbide ceramics. 17,18 Therefore, in the current work, Raman spectroscopy was used to analyze the damage beneath the static and dynamic indentations. G. Pharr—contributing editor This work was funded by a grant from the US NSF (grant# CMS-0324461) with Dr. Ken Chong as the program manager. *Member, American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: subhash@ufl.edu Manuscript No. 22412. Received October 28, 2006; approved February 20, 2007. J ournal J. Am. Ceram. Soc., 90 [6] 1850–1857 (2007) DOI: 10.1111/j.1551-2916.2007.01652.x r 2007 The American Ceramic Society 1850