Comparative analysis of fracture toughness test methods for ceramics and crystals at room and lower temperatures

Flexural and indentation tests to estimate the fracture toughness of ceramics and crystals were compared. It was demonstrated that different methods can give reliable estimates in tests of homogeneous ceramics and quite ambiguous in tests of nonhomogeneous materials. Zirconia ceramics partially stabilized with magnesia were shown to exhibit a low-temperature inelastic-elastic transition in the temperature range of their fracture toughness variations. Translated from Problemy Prochnosti, No. 3, pp. 104–118, May–June, 1997.     

Tribological Properties of Nanodiamond-Epoxy Composites

Owing to its superior mechanical properties, nanodiamond (ND) holds great potential to improve tribological characteristics of composites. In this study, we report on the wear and dry friction of epoxy-ND composites prepared from as-received and aminated ND across the length scale range from macro to nano. Comparison of macroscale, microscale, and nanoscale frictional behavior shows that ND is highly effective in improving the wear resistance and friction coefficients of polymer matrices across the different length scales. Although with both types of ND wear resistance and friction coefficients of epoxy-ND composites were significantly improved, aminated ND outperformed as-received ND, which we account to the formation of a strong interface between aminated ND and the epoxy matrix. This study also shows that agglomerates within epoxy-ND composites containing 25?vol.% ND were able to wear an alumina counterbody, indicating very high hardness and Young??s modulus of these agglomerates, that can eventually replace micron sized diamonds currently used in industrial abrasive applications.     

Strain and Fracture of a Ceramic Based on Lanthanum Chromite

The bending behavior of a ceramic — a lanthanum chromite-based solid solution La_{1 – x} Ca_xCrO₃ (x = 0.025) doped with 5 – 40 wt.% Y₂O₃ — is studied in the temperature range of 20 – 1400°C. At 20°C the composition with 5 – 10 wt.% Y₂O₃ is strained inelastically and exhibits relatively superior mechanical properties in comparison to compositions containing 20 – 40 wt.% Y₂O₃. These compositions are prone to brittle fracture within the linear elastic strain. Based on the strain and fracture characteristics of lanthanum chromite at 20°C, its mechanical behavior at high temperatures is predicted.     

Effect of Humidity on the Tribological Properties of Carbide-Derived Carbon (CDC) Films on Silicon Carbide

The effect of humidity on the tribological behavior of carbide-derived carbon (CDC) films prepared by high-temperature chlorination of silicon carbide was examined. Pin-on-disk tribological tests indicate that CDC, unlike graphite or glassy carbon, performs better in dry nitrogen (less than 0.05 friction coefficient at 0% humidity) than in humid air. This versatility is explained by the onion-like structure of the nanoporous CDC coating.     

Creep of silicon nitride-titanium nitride composites

The effect of particulate TiN additions (0–50 wt%) on creep behaviour of hot-pressed (5 wt%Y₂O₃ + 2 wt%Al₂O₃)-doped silicon nitride (HPSN)-based ceramics was studied. Creep was measured using a four-point bending fixture in air at 1100–1340 °C. At 1100 °C, very low creep rates of HPSN with 0–30 wt% TiN are observed at nominal stresses up to 160 MPa. At 1200 °C the creep rate is slightly higher, and at 1300 °C the creep rate is increased by three orders of magnitude compared to 1100 °C and rupture occurs after a few hours under creep conditions. It was established that the formation of a TiN skeleton could detrimentally affect the creep behaviour of HPSN. An increase in TiN content leads to higher creep rates and to shorter rupture times of the samples. Activation energies of 500–1000 kJ mol^?1 in the temperature range of 1100–1340 °C at 100 MPa, and stress exponentsn?4 in the stress range 100–160 MPa at 1130–1200 °C were calculated. Possible creep mechanisms and the effect of oxidation on creep are discussed.