Mechanical properties of lithic raw materials from Kazakhstan: comparing chert, shale, and porphyry

The study of lithic raw material quality has become one of the major interpretive tools to investigate the raw material selection behaviour and its influence to the knapping technology. In order to make objective assessments of raw material quality, their mechanical properties (e.g., fracture resistance, hardness, modulus of elasticity) should be measured. However, such comprehensive investigations are lacking for the Palaeolithic of Kazakhstan. In this work, we investigate geological and archaeological lithic raw material samples of chert, porphyry, and shale collected from the Inner Asian Mountain Corridor (henceforth IAMC). Selected samples of aforementioned rocks were tested by means of Vickers and Knoop indentation methods to determine one aspect of their mechanical properties: their indentation fracture resistance (a value closely related to fracture toughness). These tests were complemented by traditional petrographic studies to characterise the mineralogical composition and evaluate the level of impurities that could have potentially affected the mechanical properties. The results show that materials, such as porphyry, previously thought to be of lower quality due to the anisotropic composition and coarse feldspar and quartz phenocrysts embedded in a silica rich matrix, possess fracture toughness values that can be compared to those of chert. Thus, it appears that different raw materials cannot be distinguished from the point of view of indentation fracture resistance, calling for detailed supplementary analyses of different fracture properties. This work also offers first insight into the quality of archaeological porphyry that was utilised as a primary raw material at various Middle and Upper Palaeolithic sites in the IAMC.

Comparison of polymerization stresses of dental resin composites evaluated by two indentation fracture methods

BACKGROUND: Polymerization stress is a major problem in dental resin composite restorations. Two indentation fracture methods can be applied to evaluate the stress, however, they often calculate different values. OBJECTIVE: To compare polymerization stresses of dental composites determined by the two methods. METHODS: Glass disks with a central hole were used. Two indentation fracture methods (Methods 1 and 2) were employed to determine the polymerization stresses of low-shrinkage and bulk-fill composites. Method 1: Cracks were made in the glass surface at 300 μm from the hole. The hole was filled with the composite. Polymerization stresses at 30 min after filling were calculated from the lengths of crack extension. Method 2: The hole was filled with the composite. Cracks were introduced in the glass at 1,000 μm from the hole at 30 min after the polymerization and the stresses were calculated from the crack lengths. Stresses at composite-glass bonded interface were calculated from the stress values obtained by the two methods. RESULTS: The bulk-fill composite generated the smallest interfacial stress, and Method 1 revealed lower values than Method 2. CONCLUSIONS: The composites yielded relatively small stresses. Method 1 calculated smaller stress values, possibly affected by the lower threshold stress intensity factor.

Study of Lithium Disilicate Based Nano Glass Ceramics Containing P2O5.

Multi component Lithium disilicate based glasses containing P2O5 have been synthesized by conventional melt quenching technique. The replacement of (Li2O+SiO2) by P2O5 and its nucleating effect has been discussed. Structural features of glasses were evaluated by DTA, FTIR and Raman spectroscopy. The glass samples have been converted into glass ceramics by following three stage heat treatment schedule. XRD, FESEM, HAADF imaging and EDX analysis has been carried out for glass ceramics. Vickers microhardness and Vickers indentation fracture toughness of all the glass ceramics have been measured. UV–Visible spectroscopy study has been carried out for glass ceramics to investigate the optical properties. The glass ceramic with 1.5 mol % P2O5 has highest transmittance. Glass ceramics with P2O5 ≥1 mol % having Li2Si2O5 as main phase exhibit high Vickers microhardness (Hv) about 6.71-6.82 GPa which is suitable for dental and armor applications.

Research on Cutting Mechanism and Cutting Force Theoretical Model Based on Indentation Fracture Mechanics in Turning Lithium Disilicate Glass

Based on the theory of energy transfer, this study analyzes the fracture mechanism of machinable ceramics, and establishes the machinable ceramic tool-workpiece-chip coupling cutting force model. Research on the brittleness removal of hard and brittle materials by indentation test. The pre-existing defects on the surface of the workpiece are subjected to tensile stress. With continuous loading on a certain point on the surface of the workpiece, the cracks begin to sprout and expand; when the critical load is loaded, the cracks become unstable; with the load continues, the conical cracks expand in an unstable state. The Hertz crack system is obtained at the end of the indentation. The crack system model is established based on indentation fracture mechanics. According to Griffith energy balance theory, different types of energy models of the crack system are established. The theoretical model of cutting force is established according to the principle of energy conservation, the tool input is equal to the energy change between crack systems. The results of turning experiments show that the main cutting force increases with the increase of the radius of the tool nose, and decreases with the increase of the tool rake angle. The calculated values of the theoretical model of cutting force in turning of hard and brittle materials are basically consistent with the experimental values.

Sliding Wear Properties of HVOF Sprayed WC-10Co4Cr Coatings With Conventional Structure and Bimodal Structure Under Different Loads

The WC-10Co4Cr coatings with conventional structure and bimodal structure were sprayed by high-velocity oxygen fuel (HVOF) technology. The phase compositions and morphologies of the WC-10Co4Cr powders and coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The microhardness, porosity, bonding strength, elastic modulus, and indentation fracture toughness of the conventional coating (Conventional) and the bimodal coating (Bimodal) were also studied. The sliding wear properties of the Conventional and the Bimodal against Si3N4 counterballs under different loads at room temperature (∼25 °C) were investigated using a friction and wear tester. Compared with the Conventional, the Bimodal has denser microstructure, lower porosity, more excellent mechanical properties, and the Bimodal has better wear resistance than the Conventional under different loads. The two coatings under 15 N and 30 N only exhibit abrasive and slightly adhesive wear mechanism, while in the load application of 45 N, additional mechanism which is fatigue is detected and causes flaking of the coating.

Comparison of Different Cermet Coatings Sprayed on Magnesium Alloy by HVOF

In the present study, two different cermet coatings, WC–CrC–Ni and Cr3C2–NiCr, manufactured by the high-velocity oxy-fuel (HVOF) method were studied. They are labeled as follows: WC–CrC–Ni coating—WC and Cr3C2–NiCr coating—CrC. These coatings were deposited onto a magnesium alloy (AZ31) substrate. The goal of the study was to compare these two types of cermet coating, which were investigated in terms of microstructure features and selected mechanical properties, such as hardness, instrumented indentation, fracture toughness, and wear resistance. The results reveal that the WC content influenced the hardness and Young’s modulus. The most noticeable effect of WC addition was observed for the wear resistance. WC coatings had a wear intensity value that was almost two times lower, equal to 6.5·10−6 mm3/N·m, whereas for CrC ones it was equal to 12.6·10−6 mm3/N·m. On the other hand, the WC coating exhibited a lower value of fracture toughness.