Journal of Materials Science Research; Vol. 8, No. 3; 2019 ISSN 1927-0585 E-ISSN 1927-0593 Published by Canadian Center of Science and Education 18 Scratch Testing of Hard Ceramics: Manifestation of Viscoelasticity Witold Brostow 1, 2 , Nathalie Hnatchuk 1 & Janusz Prazuch 2 1 Laboratory of Advanced Polymers & Optimized Materials (LAPOM), Department of Materials Science and Engineering and Department of Physics, University of North Texas, USA 2 College of Mechanics and Robotics, AGH University of Science and Technology, Adama Mickiewicza, Poland Correspondence: Witold Brostow, Laboratory of Advanced Polymers & Optimized Materials (LAPOM), Department of Materials Science and Engineering and Department of Physics, University of North Texas, 3940 North Elm Street, Denton TX 76207, USA. E-mail: wkbrostow@gmail.com Received: May 5, 2019 Accepted: July 15, 2019 Online Published: July 24, 2019 doi:10.5539/jmsr.v8n3p18 URL: https://doi.org/10.5539/jmsr.v8n3p18 Abstract Viscoelasticity is studied extensively in polymers and polymer-based composites-but hardly in ceramics. We have studied viscoelastic behavior of three ceramics as manifested in scratching tests: 98 weight % B 4 C + 2% hexagonal boron nitride (h-BN), 92% B 4 C and 8% h-BN, 91% B 4 C + 5% CrSi 2 + 2% h-BN. Clear viscoelastic recovery (the groove depth becoming shallower within 2 minutes) is seen in all three materials, the recovery in each case of at least 90%. Taking into account viscoelasticity and introducing appropriate additives, one could significantly improve the scratch resistance of the boron carbides, and possibly of other hard ceramics as well. Keywords: Ceramics Viscoelasticity, Ceramics Scratch Resistance, Boron Carbides 1. Introduction and Scope Ceramic materials have a fairly large variety of applications. Thus, silver tantalite films can be used as lubricious materials for moving assemblies at temperatures as high as 750 o C (Stone et al., 2013). Kleinlogel and Gauckler (2001) have created very high density CeO 2 -allowing tailoring its electrical properties for numerous electrochemical applications. Nanocrystalline ZnO can provide low friction (Mohseni et al., 2012). Ceramic hydroxyapatite developed by Ueshima et al. (2002) can store large electric charges. Carbajal de la Torre et al. (2009) developed silica based ceramic coatings for corrosion protection of steels. Chen and Du (2014) studied a lithium lanthanum titanate as a promising solid electrolite-not experimentally but by molecular dynamics and atomistic computer simulations. Salonen and Mäkilä (2018) note applications of porous silicon in areas as diverse as luminescence, drug delivery and battery technology. Braun et al. (2018) have developed a method of fabrication of entropy-stabilized ceramics with low thermal conductivity while mechanically strong at the same time. A group at the Royal Institute of Technology in Stockholm (Larsson et al., 2000) reported scratch and indentation testing of TiN coatings on steel structures. Related to this is the development by Gonczy and Randall (2005) of an ASTM standard for Adhesion Strength and Mechanical Failure Modes of Ceramic Coatings by Quantitative Scratch Testing. Rapidly growing is application of ceramic components in human bodies in replacements of bones (St. John, 2015; Morrison et al., 2015; McEntire, 2016). One could provide still more examples, largely leading to the following conclusion: ceramic materials are being developed mostly aiming at their specific electrical, mechanical, and/or thermophysical properties. Relatively less work is devoted to tribology of hard ceramics-their behavior during scratch testing in particular. Interestingly, Singh and Shawla (2012) studied Al/SiC nanolaminates at a constant loading rate. They observed that layers of Al and SiC (each 50-60nm thick), can exhibit better scratch resistance and recovery than monolithic thin films of Al or SiC. Viscoelastic behavior means that the material shows simultaneously partly elastic (solid-like) and partly flowing liquid behavior (Lucas et al., 2001; Gedde, 2001; Brostow & Hagg Lobland, 2017). In such a material there is change of properties with time. The phenomenon manifests itself easily in polymers and polymer-based materials-and has also been seen in copper pastes (Brostow et al., 2010). When viscoelasticity manifests itself in scratch testing, the bottom of the groove created by the indenter goes up to a varying extent. The process takes place inside of 2 minutes; after 2 minutes at least 99% recovery has taken place. That recovery, so well known in polymers, has been hardly studied in ceramics.